The function of the half-bridge drive circuit:
The main function of a half-bridge drive circuit is to generate an AC trigger signal through a power transistor, thereby producing a large current to further drive the motor . Unlike microcontroller drives, which have limited driving capabilities and are generally only used as drive signals.
Working principle of half-bridge drive circuit:
Capacitors C1 and C2, along with switching transistors Q1 and Q2, form a bridge. The diagonal of the bridge is connected to the primary winding of transformer T1, hence the name half-bridge converter. If C1 = C2 at this time, then when one of the switching transistors is turned on, the voltage across the winding is only half of the power supply voltage.
The circuit works roughly as follows:
Referring to the basic topology of the half-bridge circuit, where Q1 is turned on and Q2 is turned off, the voltage applied across the transformer is half of the bus voltage, and energy is transferred from the primary side to the secondary side.
When Q1 and Q2 are turned off, the two windings on the secondary side of the transformer are in a short-circuit state because the two rectifier diodes are simultaneously freewheeling, and the primary winding is also in a short-circuit state.
Q1 is off, and Q2 is on. At this time, the voltage applied across the transformer is approximately half of the bus voltage, and energy is transferred from the primary side to the secondary side. The two diodes on the secondary side complete the commutation.
Several issues to note
biased magnetization problem
Reason: Since the potential at the connection point A of the two capacitors fluctuates depending on the conduction status of Q1 and Q2, it can automatically balance the volt-second value of each transistor switch. When the fluctuation does not meet the requirements, assuming that Q1 and Q2 have different switching characteristics, that is, under the same base pulse width t=t1, Q1 turns off more slowly and Q2 turns off more quickly, then it will affect the voltage at point B, resulting in the unbalanced volt-second values of A1 and A2 in the gray area. The reason is the turn-off delay of Q1.
If such an unbalanced waveform is used to drive the transformer, a biased magnetization phenomenon will occur, causing the iron core to saturate and generating excessive transistor collector current, thereby reducing the efficiency of the converter, causing the transistor to run out of control, or even burn out.
The waveform of a capacitor connected in series with the primary winding of a transformer.
Solution: Add a series capacitor C3 to the primary winding of the transformer. The DC bias voltage, which is proportional to the unbalanced volt-second value, will be filtered out by the capacitor. In this way, the volt-second value of the voltage will be balanced during the conduction of the transistor, thus achieving the purpose of eliminating the bias.
The selection of the two capacitors to be used as bridge arms:
From the perspective of the half-bridge circuit structure, when selecting the two capacitors C1 and C2 on the bridge arm, the voltage equalization of the capacitors needs to be considered. It's best to choose capacitors where C1 = C2. This way, when a switching transistor is turned on, the voltage across the winding is only half of the power supply voltage, achieving voltage equalization. Generally, a resistor (R1 and R2 in the schematic diagram) should be connected in parallel across each capacitor, with R1 = R2 to further meet the requirements. In this case, derating needs to be considered when selecting the resistance value and power rating. The function of capacitors C1 and C2 is to automatically balance the volt-second value of each switching transistor (the difference from C3: C3 filters out the DC component that affects the volt-second balance).
Straight-through problem
A shoot-through occurs when both Q1 and Q2 are conducting simultaneously at a certain moment, which constitutes a short circuit.
Solutions
The maximum value of the drive pulse width can be limited to prevent shoot-through at the conduction angle.
The problem can also be solved from a topological perspective by using a cross-coupled closed circuit, so that when one transistor is conducting, the other transistor is driven to be in a closed state until the former transistor is turned off, at which point the closure is lifted and the latter transistor can then conduct. This automatic blocking has the advantage of automatically adapting to storage time and parameter distribution, and the duty cycle can be used at full speed.
The selection of the two circuits mainly considers the following two points:
1. Consider the safety of the transistors based on the output voltage level; 2. Power loss issues, mainly the losses in the switching transistors and secondary windings; half-bridge circuit drive issues:
1. Primary coil overload limiting: Independent current limiting should be provided for the primary power transistor. 2. Soft start: During startup, the pulse width should be limited so that it gradually increases during the initial few cycles. 3. Magnetic control: The transistor drive pulse width should be equalized to ensure equal positive and negative magnetic flux and prevent bias. 4. Shoot prevention: The upper limit of the duty cycle should be reduced. 5. Voltage control and isolation: The circuit should be closed-loop controlled; isolation can be achieved through opto-isolators, transformers, or magnetic amplifiers. 6. Overvoltage protection: Overvoltage protection is typically achieved by blocking the converter's switching pulses. 7. Current limiting: Current limiting is installed on the input or output circuit and activates in case of a short circuit. 8. Input voltage undervoltage protection: Startup should only be possible at a sufficiently high voltage to achieve optimal performance. 9. In addition, appropriate auxiliary functions are required, such as surge current limiting and output filtering.
Driving characteristics of a half-bridge circuit:
1. The upper and lower bridge arms do not share a common ground, that is, the switching transistors of the primary circuit do not share a common ground.
2. Isolated driver.
Half-bridge driver A3909GLYTR-T http://www.dzsc.com/ic-detail/9_1601.html Specifications and parameters
series-
Output configuration half-bridge (4)
Applications include DC motors, general-purpose motors, and stepper motors.
Interface Logic
Load-type induction
DMOS technology
RDS enabled (typical) 1.6 ohms LS+HS
Current - Output/Channel 1A
Current-Peak Output-
Voltage - Power supply 4V~18V
Voltage - Load 4V~18V
Operating temperature -40°C to 150°C (TJ)
feature-
Fault protection current limiting, over-temperature
Surface mount
Packaging/Carton 10-TFSOP, 10-MSOP (0.118", 3.00mm wide) Exposed Pad Supplier Equipment Package 10-MSOP-EP Base Part No. A3909
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