Motor NotesPower Consumption when Current is Regenerated in a Parasitic Diode of a Motor Driver Output Transistor

This time, I’d like to talk about the power consumption when, during PWM driving by a brushed DC motor driver IC, current is regenerated through a parasitic diode of an output MOSFET. I have chosen this topic because the power consumption during actual regeneration is sometimes greater than the expected value obtained in calculations, and in some cases, problems requiring attention may arise.

Is the Power Consumption with Current Regeneration Through the MOSFET Parasitic Diode Greater than what a Simple Calculation Indicates?

The power consumption when current regeneration occurs through the parasitic diode of the output MOSFET should be equal to the forward voltage across the parasitic diode times the motor current. However, in actuality the power consumption is sometimes greater than this calculated value.

The reason for this is that when a current flows such that a forward voltage appears across the parasitic diode of the output MOSFET, the parasitic transistor within the MOSFET structure operates, and a current flows from the power supply to GND. This current is small, a few percent or less of the current flowing in the diode, but the power consumption equals the power supply voltage times the power supply-to-GND current, and when the power supply voltage is high, the current value cannot be ignored.

This phenomenon is explained in terms of the state of driver output MOSFETs and the flowing current, as well as the structure of the output MOSFETs.

We begin by confirming the state of the output MOSFETs during current regeneration and the flow of the regeneration current. An H-bridge circuit is shown below, but MOSFETs not related to its operation are omitted. (a) is when current is being supplied to the motor; (b) and (c) both show the circuit during current regeneration, but there are two circuit states, and so (b) represents current regeneration state 1, and (c) is current regeneration state 2.

（b） In current regeneration state 1, Q1, which had been turned on while current was being supplied, is turned off, while Q4 remains on. In this state, current is regenerated through the parasitic diode of Q2, which is turned off, and through Q4.

（c） In current regeneration state 2, Q4 and Q1, which had been turned on while current was being supplied, are turned off, and all MOSFETs are in the off state. At this time, current is regenerated through the parasitic diodes of Q2 and Q3.

Next, in order to explain the parasitic transistor that causes additional current to flow, a schematic structural diagram (cross-sectional diagram) of the output MOSFETs of the motor driver IC is shown. In the above circuit diagrams, P-channel MOSFETs are used on the high side and N-channel MOSFETs are on the low side, and so in the following diagram also, these structures are shown.

In the output N-channel MOSFET, a parasitic NPN transistor Qa is formed due to the N-type diffusion layer of the drain D, the P-type diffusion layers of the device isolation regions, and the N-type diffusion layer connected to the power supply (here, connected to the source S of the output P-channel MOSFET).

When a regeneration current flows in the parasitic diodes Di_a between the source and the drain of this N-channel MOSFET, because the device isolation P-type diffusion layer is connected to GND, a forward voltage also appears in the diode between the base and emitter of the parasitic NPN transistor Qa. As a result the parasitic NPN transistor Qa turns on, a collector current flows, and current is drawn from the power supply Ea.

In principle, this is the same for the output P-channel MOSFET. A parasitic PNP transistor Qb is formed by the P-type diffusion layer of the drain D, the back gate N-type diffusion layer that is common with the source S, and the P-type diffusion layer constituting device isolation regions and the like.

When a regeneration current flows in the parasitic diode Di_b between the source and drain of this P-channel MOSFET, the parasitic PNP transistor Qb turns on, a collector current flows, and current flows out to GND.

Thus when a regeneration current flows in the parasitic diode of an output MOSFET, the parasitic diode causes current to flow from the power supply to GND. In general, the current that flows is at least two orders of magnitude smaller than the regeneration current, but the magnitude varies greatly depending on the process of the IC and the MOSFET layout. Therefore when using a circuit in which a regeneration current flows through output MOSFET parasitic diodes, the magnitude of the current flowing due to these parasitic diodes must be checked.

For example, in the case of (c) current regeneration state 2, for a power supply voltage Ea of 24 V, regeneration current Io of 1.0 A, forward voltage of the parasitic diode of an N-channel MOSFET VF_N of 0.8 V, forward voltage of the parasitic diode of a P-channel MOSFET VF_P of 0.95 V, when the ratios to the regeneration current of the current flowing between the power supply and GND are k1 = 1/100 for both the N-channel MOSFET parasitic diode and the P-channel MOSFET parasitic diode, the power consumption Pc is as follows.

 Pc＝Io×（VF_N＋VF_P＋2×k1×Ea）
  ＝1×（0.8＋0.95＋2×1/100×24）＝1.75＋0.48＝2.23W

In this example, the effect on the power consumption of the current flowing through parasitic transistors is of a magnitude that cannot be neglected. When the power supply voltage Ea is high, this matter demands attention.