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While developing the μModule H-bridge I've learned a lot about H-bridges. Things that I have not seen written down anywhere but got burnt by them several times (literally in some cases). So I decided to share this information in the hope that it will be useful for others. Usual disclaimers apply so treat these pages as a starting point rather than a panacea.
In general an H-bridge is a rather simple circuit, containing four switching element, with the load at the center, in an H-like configuration:
The switching elements (Q1..Q4) are usually bi-polar or FET transistors, in some high-voltage applications IGBTs. Integrated solutions also exist but whether the switching elements are integrated with their control circuits or not is not relevant for the most part fot this discussion. The diodes (D1..D4) are called catch diodes and are usually of a Schottky type. Though they are mentioned in most documents dealing with H-bridges, their role is usually neglected. They are of key importance for most of the discussion on this page.
In general all four switching elements can be turned on and off independently, though there are some obvious restrictions. Though there's no theoretical restriction like that, by far the most pervasive load used with H- bridges are brushed DC or bipolar stepper motors (steppers need two H- bridges per motor).
Basic operation
The basic operating mode of an H-bridge is fairly simple: if Q2 and Q3 are turned on, the left lead of the motor will be connected to ground, while the
right lead is connected to the power supply. Current starts flowing through the motor which energizes the motor in (let's say) the forward direction and the motor shaft starts spinning. If Q1 and Q4 are turned on, the converse will happen, the motor gets energized in the reverse direction, and the shaft will start spinning in that way. If less than full-speed (or torqe) operation is intended one of the switches are controlled in a PWM fashion. The average voltage seen by the motor will be determined by the ratio between the 'on' and 'off' time of the PWM signal.
Current flow in the forward direction Current flow in the backward direction
Component selection
For the most part, the key decision to make for an H-bridge is the selection of the switching elements. There are many factors to be considered, the most important ones are the operating current, the operating voltage and the switching (PWM) frequency. For most cases a MOSFET switching element is a good selection, so for the rest of the document I will assume MOSFET switching elements.
MOSFETs, when operated as switches, have two states: on and off. In the 'on' state they have more or less behave like a small resistor. Their resistance is called channel resistance, and is denoted by rdson. Obviously the higher this value is, the higher the losses are on the MOSFET. While efficiency is not a big concern for most H-bridge designs, heat is. Since the loss on the MOSFET is converted to heat that has to be dissipated, the lower rdson is the better. Another factor to consider is that rdson is temperature- dependent and increases with temperature. Datasheets usually brag about rdson at 25oC, but that hardly can be considered as normal operating condition. So always look for rdson over the full temperature range to make sure you're operating within safe limits.
A related decision to make is to decide if 'N'-channel or 'P'-channel MOSFETs are used. 'N' channel MOSFETs have a much lower rdson values
There is another factor to be considered however: the faster the devices are switched the more sudden the voltage and current changes will be in the circuit. These changes will then generate EMI interference that is in general not a good thing. In short you don't want the switching speed (not the frequency!) to be too high or else you will generate too much electro- magnetic noise.
Switching loss is usually not that big of an issue for low-frequency (couple of hundred Hz) operation but becomes significant as frequency increases. After a certain point it is the main contributor to the dissipated heat.
P- versus N- channel high-side switches
Let's spend some time on this choice for the high-side elements. As said before, N-channel devices would be desirable for this role for their lower losses, but there's a problem: for them to operate properly, their source must be connected to the motor leads and their drain to the power rail. When a P- channel device is used, its source will be connected to the power rail and its drain to the motor leads. Now, the problem is that both devices are controlled by their gate-source voltages. For P-channel devices it means that if the gate is connected to the power supply, the device will be closed (gate-source voltage is 0) and if the gate is grounded the device is opened (provided the power-supply is actually enough to open the device), since gate-source voltage is equal to the power supply voltage.
For an N-channel device however the picture is more complicated. If you connect the gate to ground or to source, the device is closed (gate-source voltage is below or equal to 0). But where to connect it to open the device? The power supply is not enough, since, when the device is open, it's source and drain are roughly at the same potential. Since the drain is connected to power, the source will be at that potential as well, but than gate should be higher than that to keep the device open. In fact at minimum 5V higher for so-called logic-level MOSFETs and 10-15V higher for normal MOSFETs. This is a significant problem, that voltage somehow has to be generated. In most cases some kind of a charge-pump is used for that, either in a stand- alone or a boot-strapped configuration. The latter however is only useful if the bridge is driven in the 'locked anti-phase' mode (see later). In any case, these high-side drivers usually cannot deliver as much current as a regular low-side driver can, which means longer turn-on and -off times for the high- side (lower current takes longer to charge-discharge the gate-capacitance). In high-frequency operation, where switching loss is a significant factor, a P- channel MOSFET might be a better solution because of this. In low- frequency, high-current operation, where switching loss is not a problem, but channel-resistance is, N-channel transistors are usually a better compromise.
Catch diodes
Catch diodes (D1..D4) are often overlooked or just briefly mentioned in most H-bridge descriptions, but they are very important components. In fact, the
main reason for this article is to share some experience I had with H-bridges with regards to catch diodes.
The basic principle is very simple: while the bridge is on, two of the four switching elements will carry the current, the diodes have no role. However once the bridge is turned off the switches will not conduct current any more. As discussed earlier, by far the most common load for an H-bridge is an electric DC motor, which is an inductive load. What this means is that during the on-time the motor will build an electromagnetic field inside it. When the switch is turned off, that field has to collapse, and until that happens, current must still flow through the windings. That current cannot flow through the switches since they are off, but it will find a way. The catch diodes, are in the design to provide a low-resistance path for that collapse current and thus keep the voltage on the motor terminals within a reasonable range.
Now, whenever a diode is conducting current, there will be a relatively constant voltage drop on it. This is called forward voltage drop and denoted as VF. It is in the 500..1000mV range for most components. This voltage drop, combined with the current through the diode will produce some heat dissipation. The actual heat dissipation depends on the average current flowing through the diode and the percentage of the time the diode is open. As an example, if the field collapses in 1ms, the cycle time is 10ms, the current at the beginning of the off-cycle is 10A, and it drops linearly (assuming an ideal inductive load here), than there's an average 5A current flowing through the diode for 10% of the time. The dissipated heat therefore will be 5A0.5V10%=0.25W (assuming here a 0.5V VF). If however the field collapses slower, let's say in 5ms, the dissipation increases to 5A0.5V50%=1.25W. Numbers get even worse, if the field doesn't completely collapse by the end of the off-cycle. From these numbers it can be seen that the dissipated heat over the diodes can be in the same ballpark as the heat dissipated on the switching elements.
It is important to see that for at least low-frequency operation, the selection of the switching elements is mainly determined by the maximum static current delivered by the bridge, while the selection of the diodes is more involved and includes analysis of the actual collapse of the inductive field. The diode selection will be dependent on the dynamic behavior of the bridge. The design and the control of the bridge has a significant effect on how long the diodes actually conduct, so with the one type of control the bridge can actually survive higher currents than with another. With some driving modes, the diodes almost don't conduct at all.
One important feature of MOSFET transistors is that they contain an intrinsic (unavoidable, built-in) diode between their drain and source. This diode acts as a catch diode in an H-bridge configuration, and most MOSFET datasheets specify the parameters of this diode. It is thus possible to use this built-in diode of the transistors and not provide external ones if the specification of this diode meet the design requirements. For bipolar transistors there's no such intrinsic diode so external diodes always have to be provided.
motor will generate torque but the voltage on the motor terminals will be 0 (or close to 0 if we consider losses as well).
Though we will in most cases ignore motor losses in the following discussions, or consider them constant, in many cases motor losses are dependent on the speed. We will also consider Lm, Rm and Vg to be constant at a given speed, at least for the most part. In fact however due to the commutation in the motor, and the conducting winding relative position to the stators' magnetic field, all of these values are a function of the rotor position as well. This will become important when we'll discuss back-EMF measurement.
Drive modes
A bridge can be driven in many different ways. In general, the on-time behavior is rather simple: you have to turn on one high-side and the opposite low-side switch to allow current to flow through the motor. It is the off-time drive that makes a difference. Since Q1 and Q2 (or Q3 and Q4) should never ever be turned on at the same time, there's only three different combinations for those two switches. Either Q1 conducts, or Q2 conducts, or none. In the following diagrams I will use a simplified drive notation: low level means that the low-side is turned on (Q2 or Q4). A mid-level denotes the state where neither of the switches conduct, while high means that the high-side is turned on (Q1 or Q3). It is important to note that actual drive voltages depend on the component selection ('P' or 'N'-type high-side MOSFETs), and that two independent driving signal will have to be generated for the two transistors:
Symbolic and actual drive signals for 'N'- type high-side MOSFETs
Symbolic and actual drive signals for 'P'- type high-side MOSFETs
Continuous and discontinuous current
This is actually a switching power supply term, but in many ways H-bridges and step-down power supplies are quite similar. It denotes two significantly different operating modes of the bridge. Whether the current in the motor reaches 0 during the off-time or on. If it does, we're talking about discontinuous current mode, if it does not, continuous current mode. The distinction is important for many reasons, among other things the dissipated power on the catch diodes will be different. The ratio between the max. current and the average current on the motor will always be greater than two for discontinuous and lass then two for continuous mode operation. From this standpoint, continuous mode is preferred. On the other hand, whenever the current drops to 0, the voltage on the motor terminals will be Vg (no voltage drop on the resistor or the inductor), which can be used to measure the speed of the motor.
Whether the circuit operates in continuous or discontinuous mode depends on the drive mode, the load of the motor (more precisely the speed of the motor) and the power supply voltage.
In the following discussion I will assume discontinuous operating modes. The calculations can be easily repeated for continuous mode as well. Also, I will always assume that during on-time Q2 and Q3 are conducting, in other words, the motor is energized in the forward direction.
Sign/Magnitude drive
This is the simplest drive mode. During on-time (as with all other drive modes) one high-side switch and the opposing low-side switch is open, the other two are closed. The motor current increases during this period from 0 to its maximum value.
During the off-time, the high-side MOSFET stays on, while the low-side is turned off. The motor current will continue to flow through Q3 and D1. It cannot flow through D2 since the forward current on D2 is in the opposite direction to the motor current (in other words D2 will never be forward-biased in this mode). The voltage on the motor terminals will have to be VF for this. The voltage on the motor coil will be Vg+VF-IRm, or approximately Vg, disregarding Rm. If the motor is under no load, than Vg is approximately Vbatton/tcycle. If the motor is stalled, Vg is 0. Since the current change on the inductor is proportional to the inductor voltage (VL=L*dI/dt), in the no load case, the current will decrease very slowly, while in the stalled case, it will decrease in approximately the same rate as it increased.
Once the current reaches 0, D1 closes, and the generator voltage Vg appears on the motor terminals. The circuit remains in that state until the next cycle begins.
element for the collapse current. The collapse time and other operating parameters of the circuit is roughly the same:
It is usually a good idea to switch the high-side element as few times as possible, since their turn-on transients are slower, and thus their switching losses are higher. In that sense the first drive mode preferred. However if the operating frequency is low enough that switching loss is not an issue, one can equalize the power-dissipation on D1 and D4 by alternating the two drive-modes. This trick can cut dissipated heat on each of diodes in half and can very well move them to the safe operating range. At any rate, higher average and peak current can be achieved with this operating mode, provided that the diodes are the limiting factor.
Lock anti-phase drive
This rather popular drive mode removes almost all stress from the catch- diodes. In this mode, the motor is energized in the reverse direction during the off-time. In other words during the on-phase Q2 and Q3 are conducting while in the off time Q1 and Q4 are on. The diodes never carry current except for the short period of switching the transistors.
During the off-time the voltage on the motor windings is roughly Vbat+Vg, significantly higher than for the previous drive modes. This results in much faster collapse of the field. The problem however with this drive mode is that once the current reaches 0, it continues to decrease, into the negative values. At this point the motor is energized in the reverse direction, effectively trying to turn the shaft in the wrong direction. Another characteristic of this drive is that the generator voltage (Vg) never appears on the motor terminals. It is not a big problem for traditional motor driver circuits however if back-EMF speed-control is to be used, this drive mode is not suited for it.
Active field-collapse drive
This is a variation of the above idea: during the off-time connect the battery in the reverse direction to the motor, so that the collapse of the field is faster, however don't let the motor current to become too negative.
In this mode, during on time we have the usual Q2 and Q3 conducting, but in the off-time, we turn off both of them, and turn on Q1. This result in D becoming forward-biased, opening and start conducting current. The motor
Modified active field-collapse drive
If it is possible to measure motor terminal voltages in the circuit, some modifications can be made to the above drive mode to make it more efficient. As it can be seen from the previous diagrams, when the motor current reaches 0, the 'B'-side motor terminals voltage jumps from ground to Vbat+VF. If the circuit can detect this transition and turn Q1 off, and Q2 on, the motor will not open D3 any more, Vg can appear on the motor terminals, and the current remains 0.
This drive-mode removes all stress from D3, the only conducting diode will be D4. However it inherits the fast collapse of the motor current from the previous design, and so it dissipates significantly less power than the sign/magnitude drive modes.
One interesting characteristics of the active-collapse drive modes (both the original and the modified) is that the collapse current flows through the battery. What it in effect means that during the off-time the collapse-current charges the battery back. While in general this is a good thing, it has to be ensured that the battery is be able to sink that current, otherwise Vbat starts
rising potentially to dangerous levels. If the battery cannot sink the energy pumped back by the collapse-current, a large capacitor must be connected to the power supply to damp the battery voltage-increase. The exact value of this capacitor can be calculated from the amount of charge the collapse- current delivers (Imax*tcollapse/2) and the maximum allowed battery voltage increase. In general this will be a rather large value.
Of course the same technique can be done on the low-side, using Q4 and D1 for the collapse-current. It is also possible to alternate Q1 and Q4 drive- modes, thus splitting the diode-load in half.
Synchron collapse drive
If one can measure the current through the motor, and accurately detect when it crosses 0, it becomes possible to modify the lock anti-phase drive in a different way. In this case, during the off-phase Q1 and Q4 are conducting, but only until the field collapses. At that moment, both Q1 and Q4 are turned off and instead Q2 (or Q3) are turned on. This will allow Vg to appear on the motor terminals, but none of the diodes will be forward-biased so the current remains 0 for the rest of the off-cycle.
generator voltage, than closed-loop speed-control of the motor can be achieved without any external measuring element. As I described it previously, any time the motor current is 0, the generator voltage appears on the motor terminals. Certain drive-modes have a period in their cycle where the current is 0. Those drive-modes can be used in conjunction with this technique, the generator-voltage can be sampled and the control loop can be closed. These drive modes are the sign/magnitude drive , the modified active collapse drive and the synchron collapse drive. For measuring back-EMF, the voltage of the motor terminals has to be measured, so the all the required circuitry for the modified active collapse drive is there anyway. However for the synchron collapse drive, the motor current has to be measured as well.
It is important to note that the generator-voltage measurement can only be done if the motor current does in fact reaches 0, so the circuit must operate in the discontinuous current mode. Since the average current in this mode cannot be more than half of the short-circuit current, to reach maximum torque, a higher than nominal battery voltage has to be used. Whether the circuit operates in continuous or discontinuous mode, depends on many things, among other things, the generator voltage itself. This poses a problem, so the sampling circuit has to be able to detect if the circuit is in deed in discontinuous mode. It is even more important if the circuit operates with a higher than nominal battery voltage, since that, in continuous current mode can damage the motor.
There are many ways of capturing the voltage on the motor terminals. You can measure the differential voltage, or measure the voltage on the two terminals individually. It is interesting to note that one of the leads is always connected to either the positive supply rail or to ground, so it is enough to measure the voltage on one of the leads, since the other terminal is at a known potential.
Measurements
The following screen captures show actual waveforms measured on the leads of a motor, while driving in different drive-modes:
Sign/Magnitude drive with high-side drive
Sign/Magnitude drive with alternating high- and low-side drive
Active-collapse drive with alternating high- and low-side drive
Those wavy parts are where the generator voltage is visible on the motor poles.
Complications
Those waves lead us to this chapter: why are they there and what can we to about them. It is obviously a problem to measure the generator voltage if they're so noisy.
To identify the route cause we have to go back to how DC motors work. In an average small DC motor, you'll find a permanent magnet in the stator (the non-moving shell of the motor) and three coils in a three pointed star arrangement on the rotor (the moving part of the motor). It also has two commutators that connect the external wires to the coils:
If you need an introduction to how these motors work, see http://www.phys.unsw.edu.au/~jw/HSCmotors.html or http://www.solarbotics.net/starting/200111_dcmotor/200111_dcmotor2.html for an excellent explanation.
As the rotor rotates the relative motion of each coil to the magnetic field changes in a sinusoidal fashion. The commutator selects a different coil depending on the position of the rotor, so the voltage on the motor terminals is always as high as possible:
