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MOS Lamp Drive Circuit Design Details Engineers Should Know


It is generally accepted that MOSFETs are voltage controlled and do not require any drive current. However, there is junction capacitance between G and S stages of MOSFET, and this capacitance will make driving MOSFET not so easy.

MOS Lamp Drive Circuit Design Details Engineers Should Know

Apart from requirements such as ripple and EMI, faster switching speed of a MOS lamp, better, because shorter switching time, lower switching loss, and switching loss accounts for a large proportion of total loss in switching power supply, so quality of MOS tube drive circuit directly determines efficiency of power supply.

For a MOS lamp: if time required for voltage to change between GS from 0 to lamp turn-on voltage is shorter, faster MOS lamp will turn on. Similarly, if time of voltage drop GS of MOSFET from turn-on voltage to 0 V is shorter, faster MOSFET will turn off.

From this we can learn that if we want to increase or decrease GS voltage in a shorter time, we need to apply more instantaneous drive current to gate of MOSFET.

The commonly used PWM chip output directly drives MOSFET or is amplified by a triode and then drives MOSFET. In fact, instantaneous excitation current has a big drawback.

The best method is to use a custom MOSFET driver IC like TC4420 to drive MOS lamp. This type of chip usually has a large instantaneous output current and is also compatible with TTL input level. The internal structure of MOSFET driver chip is as follows:

MOS Lamp Drive Circuit Design Details Engineers Should Know

Attention when designing MOSFET driving circuits:

Because there will be parasitic inductance in control circuit wiring, and parasitic inductance and junction capacitance of MOSFET form an LC oscillation circuit. If output pin of driver IC is directly connected to gate of MOSFET, PWM waveform will rise. The falling edge will generate large oscillations, causing MOS tube to heat up or even explode. A common solution is to put a resistor of about 10 ohms in series with gate to reduce Q value of LC tank so that oscillation decays quickly.

Due to high input impedance of MOSFET gate, a little static electricity or interference may cause MOSFET to conduct improperly, so it is recommended to connect a 10kΩ resistor in parallel between MOSFETs G and S. to reduce input resistance.

If you are concerned that interference on a nearby power line will cause an instantaneous high-voltage breakdown of MOSFET, you can connect a TVS transient suppression diode of about 18V in parallel between GS.

TVS can be thought of as a high response voltage regulator tube that can handle hundreds of kilowatts of power instantaneously and can be used to absorb instantaneous interference pulses.

Link to MOS lamp drive diagram:

MOS Lamp Drive Circuit Design Details Engineers Should Know

MOSFET drive circuit wiring diagram:

The circuit area of ​​the MOS lamp driving circuit should be as small as possible, otherwise external electromagnetic interference may be introduced.

The bypass capacitor of driver chip must be as close as possible to VCC and GND pins of driver chip, otherwise wiring inductance will greatly affect instantaneous output current of chip.

MOS Lamp Drive Circuit Design Details Engineers Should Know

General drive waveform of a MOS lamp

MOS Lamp Drive Circuit Design Details Engineers Should Know

If there is such a round waveform, just wait for a nuclear explosion. Most of time lamps operate in linear region and losses are extremely high.

In general, this situation is such that wiring is too long and inductance is too large, and grid resistance does not help, so you can only redraw the board.

MOS Lamp Drive Circuit Design Details Engineers Should Know

Very disfiguring square wave with high frequency ringing.

The vibration is strong on rising and falling edges, in this case tube usually goes out instantly, which is similar to previous case and goes into a linear region.

The reason is similar, mainly a wiring problem. Thick and round thick pork waves.

The rising and falling edges are very slow due to impedance mismatch.

The control capability of IC is too low or gate resistance is too high.

Resolutely change high current driver chip, and adjust gate resistance to a small adjustment.

The swollen face is filled with a sine triangular wave, born in square wave family.

The resistance of drive circuit is too high. This is obligatory wave of Guangzi. The solution is same as above.

MOS Lamp Drive Circuit Design Details Engineers Should Know

Popular face shape, square wave that everyone loves.

The top and bottom levels are different, and level can currently be called a level because it is flat. The edge is steep, switching speed is fast, loss is very small, and small fluctuations are acceptable. The tube cannot enter linear region. If you have an obsessive-compulsive disorder, you can increase the mesh resistance accordingly.

MOS Lamp Drive Circuit Design Details Engineers Should Know

Straight beautiful waves, no ringing, no peaks, no loss of linearity, three-no products, this is most perfect waveform.