The technical characteristics of unipolar and bipolar PWM are compared, highlighting the limitations of current half-bridge driver ICs. A unipolar PWM logic distribution circuit is designed using a simple logic gate. After amplification by a half-bridge driver IC, an H-bridge power conversion circuit composed of IGBTs is used to drive a servo motor for radar antenna control. This circuit design addresses the issue of dynamic bootstrap, enhancing both the speed of the radar antenna and the efficiency of the power conversion system.
With the advancement of high-power semiconductor technology, pulse width modulation (PWM) using fully controlled power electronic devices has become widely adopted in radar antenna control systems. These systems typically use PWM to regulate motor speed, with H-bridge circuits made of power transistors commonly used to drive servo motors. Based on the polarity of the voltage applied across the motor’s armature during one switching cycle, PWM can be classified into either bipolar or unipolar modes.
In a bipolar PWM converter, the control signals of the upper and lower arms on the same side are opposite PWM signals, while the control signals between different sides are the same. When the duty cycle is 50%, the motor remains stationary, but the instantaneous voltage and current at the armature terminals alternate, resulting in zero average current. This causes high-frequency micro-vibration, which helps eliminate friction dead zones. However, at low speeds, the pulse width of each power transistor remains wide, ensuring reliable conduction. Despite this, all four power transistors are in switching mode during operation, leading to significant switching losses and a higher risk of “straight-through arm†faults. Moreover, the motor’s armature is not purely inductive, and when it is stationary, alternating current still flows, causing energy loss due to internal resistance and reducing overall conversion efficiency.
In contrast, a unipolar PWM converter operates such that the upper and lower arms on one side alternate between positive and negative pulses, while the upper arm on the other side is turned off and the lower arm remains on. This eliminates the need for alternating conduction during operation, reducing switching losses. At a 0% duty cycle, the motor stops, and the H-bridge is completely turned off, preventing any current flow. This results in no energy consumption from the motor’s internal resistance. Additionally, the armature voltage in a unipolar configuration is less than half of that in a bipolar setup, leading to reduced rotational speed fluctuations. However, both unipolar and bipolar PWM configurations require a logic delay to prevent “straight-through arm†conditions.
When designing an H-bridge power conversion circuit, one key challenge is addressing the high-side gate suspension drive. Common solutions include using pulse transformers for isolation, employing independent floating power supplies, or implementing dynamic bootstrap technology. While the first two methods are effective, they often involve numerous discrete components, increasing complexity, reducing reliability, and expanding PCB size. Dynamic bootstrap technology, widely used in dedicated circuits, offers high integration, compact size, and stable performance, allowing single-supply operation. However, it requires a bootstrap diode and capacitor, along with a suitable charge and discharge circuit, to form a dynamic bootstrap loop. This process must be cyclic to ensure proper turn-on and turn-off of the high-side gate. The unipolar PWM circuit described here effectively solves these issues.
1. H-type Unipolar PWM Design
1.1 Pulse Distribution Circuit Design
A unipolar PWM pulse distribution circuit was designed, as shown in Figure 1. The input consists of a direction signal and a PWM signal, which are pulse-distributed to generate a unipolar PWM output. The signal ground and power ground are isolated using a high-speed optocoupler. The duty cycle of the unipolar PWM can be adjusted by varying the PWM signal’s duty cycle. The direction signal controls the motor's rotation direction, differing from the bipolar PWM approach, where direction is determined by the duty cycle. Notably, the NE555 circuit acts as a pulse detector, ensuring the H-bridge is turned off if the PWM signal is lost. Simulated waveforms are shown in Figure 3.
1.2 Drive and Power Conversion Circuit Design
The unipolar PWM signal generated by the pulse distribution circuit is amplified by a half-bridge driver, as shown in Figure 2. The IR2308 from International Rectifier is used alongside an H-bridge composed of IGBTs. During high-side gate driving, a bootstrap diode and capacitor are required. When the low-side IGBT and motor load pull the Vs pin to ground, the bootstrap capacitor charges from DC +18 V through the bootstrap diode. When the high-side IGBT turns off, the capacitor discharges through the HO pin, enabling the IGBT to saturate. The internal deadband protection in the IR2308 provides necessary delay, preventing “straight-through arm†faults. Since the low-side IGBT is always on, the bootstrap capacitor can be charged through the motor load, stabilizing the bootstrap voltage and reducing fluctuations.
1.3 Calculation of Bootstrap Components
Bootstrap component selection significantly affects the performance. The minimum charge required from the bootstrap capacitor is given by:
Qbs = C × (Vcc - Vf - VLS - VMin)
Where Vcc is the logic circuit supply voltage, Vf is the forward voltage drop of the bootstrap diode, VLS is the voltage drop across the low-side IGBT, and VMin is the minimum voltage between the gate and source. Leakage current from electrolytic capacitors should be considered, so non-electrolytic capacitors are preferred. The bootstrap diode must withstand the bus voltage and have fast recovery characteristics to minimize charge loss. Recommended specifications include a maximum reverse voltage ≥ Vbus, reverse recovery time ≤ 100 ns, and forward current ≥ Qbs.
2. Experimental Verification
2.1 Experimental Methods and Device Parameters Selection
In this experiment, a TMS320LF2407A DSP from TI generates a 20 kHz PWM signal with adjustable duty cycles from 0% to 90%. A 100 V/2 A DC servo motor with an armature resistance of 8.1 Ω is used. The H-bridge bus voltage is set to 100 V, with a maximum switch current of 2 A. The IGBT used must have a collector-emitter voltage rating above 100 V and a collector current rating above 2 A. For inductive loads like motors, freewheeling diodes are connected to each H-bridge switch to absorb counter electromotive force. The Fairchild IGBT FGAT25N120 meets these requirements. Calculations show that the bootstrap capacitor must provide at least 612.5 nC of charge, requiring a 220 nF ceramic capacitor.
2.2 Actual Application Effect on Radar Antenna
As shown in Figure 2, the average armature voltage for a bipolar H-bridge PWM motor is given by:
UAB = Ï„(Vbus - 2VCE(sat))
At Ï„ = 0%, UAB = 0 V, and the motor stops. All IGBTs are off, eliminating switching loss and energy consumption. At Ï„ = 90%, the motor runs at a high speed, with an average current of 1.72 A. The calculated armature voltage is 86.4 V, and the power input is approximately 148.61 W. The copper loss is 23.96 W, leaving 124.65 W as electromagnetic power, resulting in a conversion efficiency of around 83.8%. Compared to bipolar PWM, the unipolar version reduces switching and copper losses, improving efficiency and speed.
3. Conclusion
The H-type unipolar PWM circuit overcomes the inefficiencies of bipolar PWM, especially when the motor is stopped or running at low speeds. It reduces power consumption, enhances conversion efficiency, and increases the radar antenna’s speed. While similar integrated H-bridge drivers exist, such as National Semiconductor’s LMD18200, they typically operate at lower voltages and are more expensive. The proposed circuit achieves unipolar PWM functionality with a simple logic addition, allowing bus voltages up to hundreds of volts.
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