PWM-based light dimmer/speed controller

A few months ago I modified a window fan to use a light dimmer as a speed controller. I mentioned that at low speeds, a hum could be heard from the motors since the voltage fed to the fan was not a sine wave. I removed the speed control circuit built into the fan and replaced it with an external light dimmer. When the fan was running at full speed, it drew about 70 watts and had a near-100% power factor. However, as the dimmer was reduced to a lower setting, the motors got louder and the power factor dropped.

In this article, I will describe a PWM-based dimmer that maintains a near-100% power factor no matter the speed setting of the dimmer. The schematic is shown below:

gEDA schematic file | Link to simulation

A 555 timer (U1) is used to generate the PWM signal and drive the gate drive transformer (T1). Suppose the capacitor C4 is charged at 1/3 Vcc. The 555 timer switches its output on, and the capacitor is charged through D4 and the upper half of the potentiometer. When the capcitor reaches 2/3 Vcc, the 555 timer swithces its output off, and the capacitor is discharged through D2 and the lower half of the potentiometer. As the resistance of the top half of the potentiometer increases, the resistance of the bottom half decreases. Thus, as the on time increases, the off-time decreases, which keeps the frequency nearly constant.

The output of the 555 timer drives the gate drive transformer via C2 and R3. R3 acts as a current limiting resistor, and C2 blocks the DC component of the output of the 555 timer. However, because the capacitor blocks DC and the transformer (which can act as an inductor) shorts DC, as the duty cycle of the output increases, the voltage while on decreases. This effect is shown below:

To solve this problem, capacitor C3 and diode D7 were added to the secondary of the gate drive transformer. When the output of the 555 timer is low, the output of the gate drive transformer is negative, so the diode D7 charges capacitor C3. Additionally, since the gate of the MOSFET acts as a capacitor, is will remain high since it is unable to discharge through D8. However, it can discharge through R4 and the base-emitter junction of Q2. Q2 will switch on and quickly discharge the gate of the MOSFET so it does not saturate. When the output of the gate drive transformer is positive, the voltage on the capacitor C3 adds to the voltage on the transformer. Thus, as the negative peak voltage decreases, the voltage on the C3 increases, and the amount added to the positive peak increases. The peak voltage after C3 will therefore remain constant. Additionally, the gate of the MOSFET is charget via diode D8 and C3. The voltage after capacitor C3 is shown below:

Diodes D1, D3, D5, and D6 form bridge rectifier with the AC inputs connected to L_IN and L_OUT and the DC outputs connected to the MOSFET. This will ensure that the drain of the MOSFET is always at a higher voltage than the source. When the MOSFET is on, current either flows through D1, the MOSFET, and D5, or through D3, the MOSFET, and D6, depending on the direction of the current. Thus the L_IN and L_OUT terminals are effectively shorted together. By applying a PWM signal to the gate of the MOSFET, the voltage at the load is driven with the same PWM signal, so the average voltage can be decreased.

In my case, the gate drive transformer is contructed from a used flyback transformer. The transformer originally had a primary (which was made of 4 sections) winding, a secondary winding, and an auxiliary winding. The secondary and auxiliary windings were removed, as well as two of the four primary windings. The remaining two sections of the primary were cut apart, and these windings were used as the gate drive transformer. The core was not replaced afterwards, since it had been broken while removing it. A previous version used a toroidal transformer, however the coupling was too poor.

Results

The switching frequency of the device was measured to be around 350kHz. The gate drive waveform had a rising edge lasting 100ns and a falling edge lasting 50ns. Although the gate voltage was initially around 12 volts at the beginning of the on-time, it drops to around 7 volts before switching off. (TODO: image)

When the dimmer was tested with an incandescent light bulb, the MOSFET remained cool and the power factor remained near 100% no matter the setting of the dimmer. When the dimmer was tested with the window fan, a 100nF capacitor was needed across the output to make the fan run. The power factor was still above 90%, but the MOSFET would heat up even when attached to a heatsink. I suspect that this may be because of slow rising and falling edges on the gate drive circuit. However, the MOSFET was heating up quite fast assuming it was only dissipating power as it was switching.

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