The performance of the drive circuit greatly affects the overall system performance. There are many issues that need to be carefully designed, such as turn-on delay, pump-up protection, overvoltage and overcurrent protection, switching frequency, and selection of additional inductors. 1. Selection of switching frequency and additional inductor in the main circuit Torque fluctuation is also known as current fluctuation. The torque fluctuation index given by the system design is ΔI/IN. For brushed DC motors, it is usually around (5~10)%. For ease of analysis, we can assume that ΔI/IN=ΔI/(Us/Rd) (1) where Rd is the total resistance of the armature circuit. Substituting into the ΔI expression of various drive control methods above and eliminating Us, we can find: For unipolar control, Ld/Rd≥5T～2.5T (reversible or irreversible) (2) For bipolar control, Ld/Rd≥10T～5T (3) where T is the switching period of the power switch. For brushed DC motors, the electromagnetic time constant Ld/Rd is generally between 10ms and tens of milliseconds. If GTR is used, the switching frequency can be around 2KHz, T=0.5ms. If IGBT is used, the switching frequency can be above 18KHz, so the above formula can be satisfied. If GTO or thyristor power devices are used, since the working frequency is only around 100Hz, a reactor should be added to the main circuit, and Ld="Lf"+La (4) For irreversible systems, the critical current should be further checked, IaL=UsT/8Ld≤Ia0 should be less than the motor no-load current to prevent no-load runaway. For low inertia motors and torque motors, since the electromagnetic time constant is very small (a few milliseconds or less), IGBT power switching devices with high switching frequency should be considered. 2. Selection of power drive circuit [align=center] Figure 1 H-bridge switch circuit (Ⅰ) Figure 2 H-bridge switch circuit (Ⅱ) [/align] The low power drive circuit can use the H-bridge switch circuit shown in Figure 1. UA and UB are complementary bipolar or unipolar drive signals, TTL level. The withstand voltage of the switching transistor should be greater than 1.5 times Us. Due to the high cost and difficulty in implementing high-power PNP transistors, this circuit is only used in low-power motor drives. When all four power switches use NPN transistors, the base level offset of the two upper bridge arm transistors (BG1 and BG3) needs to be addressed. The H-bridge switching circuit in Figure 2 utilizes two transistors to achieve the level offset of the upper bridge arm transistors. However, the loss on resistor R is significant, so it can only be used in low-power motor drives. When the drive power is relatively high, the bridge arm voltage is generally also relatively high, for example, directly using the power frequency voltage, single-phase 220V, or three-phase 380V. For safety and reliability, it is desirable for the drive circuit (main circuit) to be isolated from the control circuit. In this case, the main circuit must use a floating ground pre-drive. The floating ground pre-drive circuits shown in Figure 3 are all independent and powered by independent power supplies. Because optocouplers are used in the pre-drive circuits, the control signals are electrically isolated from their respective pre-drive circuits, thus making the control signals float (or not share a common ground) with the main circuit. [align=center]Figure 3 High-Power Drive Circuit[/align] 3. Pre-Drive Circuit with Optocoupler Insulation For high-power drive systems, it is desirable to achieve electrical isolation between the main circuit and the control circuit. Optocoupler circuits are often used to achieve this. Three commonly used optocoupler circuits are shown in Figure 4. Typical models of the ordinary type are 4N25 and 117, typical models of the high-speed type are 985C, and the high current transfer ratio type, also known as Darlington type, is typically 113. [align=center]Figure 4 Typical Optocoupler Circuit[/align] In the figure, the Ic/Id of the ordinary optocoupler is 0.1～0.3; the high-speed optocoupler uses a photodiode; the Ic/Id of the high current transfer ratio optocoupler is 0.5; their rise delay time and turn-off delay time are tr,ts>4～5μs; tr,ts<1.5μs; and tr,ts is about 10μs, respectively. The optocoupler, combined with subsequent circuits, can form a pre-drive circuit, as shown in Figure 5. The rise delay tr of this pre-drive circuit is 3.9μs, and the turn-off delay ts is 1.6μs, making it suitable for use in medium-power systems. [align=center]Figure 5 Pre-drive circuit[/align] To provide optimal pre-drive for power switches, many dedicated pre-drive modules are now available. These modules provide an ideal pre-drive signal to the power switch, ensuring rapid turn-on and turn-off, optimal control of the power switch's saturation depth, and detection and protection against overcurrent and overheating. Examples include EX356 and EX840. 4. Anti-shoot-through turn-on delay circuit Applying complementary signals to the upper and lower bridge arm power transistors of the H-bridge drive circuit can cause a "bridge arm shoot-through" fault if, for example, the lower bridge arm transistor fails to turn off in time while the upper bridge arm turns on first, due to the load conditions, the transistor's turn-off time is usually longer than its turn-on time. During bridge arm shoot-through, the current increases rapidly, damaging the power switch. Therefore, setting a turn-on delay is essential. Figure 6 shows the turn-on delay circuit and its waveform. [align=center] Figure 6 Turn-on delay circuit and waveform[/align] The turn-on delay, sometimes called the dead time, can be set by the RC time constant; for GTR, it can be set to 0.2μs/A; for MOSFET, it can be designed to be 0.1~0.2μs and is independent of the current; for IGBT, it can be designed to be 2~5μs. For example, if it is a GTR, f=5kHz, bipolar operation, the pulse width modulation range is T/2=1/10=0.1ms. If I=100A, then Δt=0.2X100=20μs, then the maximum possible PWM modulation resolution is (T/2)Δt=0.1/0.02=5 (5) This shows that the dead time occupies 1/5 of the modulation period, which is obviously not feasible. Therefore, for a 100A motor system, the switching frequency of GTR must be lower than 5kHz. For example, below 2kHz, the resolution is about 12.5. There are still many issues in the design of the drive circuit, such as overvoltage, overcurrent, overheating, pump-up protection, etc.