24V H bridge, pwm Frequency problem

asunca · Joined: 09 Sep 2005 Posts: 36

i ve got a 24 V dc motor in my h-bridge. the gate input voltage is 30V driven on a 147ohm collector resistor(5watt) and BD139 power transistor (to be able to increase the current to fill the gate capacitance). but altough it is recommended to use the pwm input at 1-5 Khz approximately, my motor starts to whine at about 100Hz. i think mosfets are not working fast and they dont respond to high frequency. for charging an discharging i am giving approximately 1A but still no way??this still insures gate capacitance cannot charge and discharge quickly. what should i do??? i am tired of hearing the whining!!!! thanks

Ttelmah · Guest

Use a MOSFET driver IC, like the TC4422, 4427 etc..
MOSFETs, need a _lot_ of instantaneous drive current. Typically, their internal structure, is like thousands of conventional FETs in parallel. FETs that only have perhaps 0.5pF gate capacitance each, but once you have over 1000 in parallel, gate capacitances in the order of 1000pF, are common. Resistive drives, have the problem, that the current actually delivered, falls as the voltage on the gate approaches the drive voltage, so you either have to have a accept slower switching times than are optimal, or deliver a lot more current than is comfortable...
Also allow some dead time between the sides of the H-bridge (you will get some very nasty current flows, if both FETs on one side of the bridge are on at the same moment, which can happen if the 'switch on' time of your driver is faster than the 'switch off' time). How are you snubbing the inductance?. What trap diodes are you using? (unless your FETs are specifically designed for this, do not rely on the internal diode in the FET).

Best Wishes

Brian S · Joined: 06 Sep 2005 Posts: 13

Many motors show acoustic resonances in the audible range. These are very hard to PWM without whining. A higher PWM frequency can help, as can bad hearing....

As Ttelmah noted, the 4426 series drivers are a good solution, as they can turn the FET gates on AND off in the low nS range.

A driver subsystem that works well at 24V+ is the Vishay SI9978. It includes non-overlap and automatic soft-start/current limit circuitry, and runs an "all-NChannel" bridge.

I've run both drivers at 19KHz with no whining or smoke, using avalanche-rated FETs.

MikeValencia · Joined: 04 Aug 2004 Posts: 238 Location: Chicago