555 drives gate of IRFZ44 slowly. Does 555 have enough current to control the gate properly?

Question

I'm using a 555 for PWM at 2.4 kHz to control a DIY brushless motor. The output of the 555 is going to the gate via a 1 kΩ resistor. At 240 Hz, the waveform has a flat top. At 2.4 kHz, it's rising slowly. I thought the 555 had enough current control the gate better at much higher frequencies than 2.4 kHz. I thought it could be too high of a resistance to the gate at 1 kΩ, but the waveform looks the same on both sides of the resistor. If the resistor was limiting the current too much, wouldn't the waveform be square before the resistor and rounded after? Do I really need a gate driver to get fast switching using a 555 and IRFZ44?

Answer 1

At 2.4KHz, it's rising slowly.

I see no problem. The top does not need to be flat. It's not the flatness of the top that matters. It is the slew rate of the rising and falling edge, measured in V/us. That's what you should be checking, since that's what drives losses.

To see what's going on you probably need to set the sweep rate between 1μs/div and 10μs/div. Much, much smaller timescale than what you're looking at as shown on the oscilloscope shot.

Do I really need a gate driver to get fast switching using a 555 and irfz44?

How do you know that your switching is not fast enough when you have not shown any waveforms that are on a timescale short enough to actually determine that?

A "gate driver" can be as simple as a complementary emitter follower attached to the output of the 555.

^{simulate this circuit – Schematic created using CircuitLab}

C1 should be ceramic.

Answer 2

The IRFZ44 has a total gate charge of 63nC. The 555 output can source at least 100mA of current. This gives a rise-time of 63nC/100mA = 630ns which should rapidly turn on the MOSFET.

For maximum rise-time use no more than 10Ω in series with the gate.

Here is the LTspice sim of a 555 driving the MOSFET. It shows a rise-time of <500ns.