need help on snubber ciruit

[xinzc28]

I'm building stepper driver using L297+L6203.
since the snubber circuit on L6203 datasheet seem not operate very well, I need to redesign it. I found an application note with snubber formula.

my stepper motor current rate is 3 amp.
my power supply is 24volt.
my chopper rate is 25880 Hz (~26 Khz)

1st equation: (done)
Rmax = Vs min / I peak = 22.5 / 3 = 7.5 ohm

2nd equation:
C = I peak * dt/dv --> how to determine my dt & dv ???

3rd equation:
Pd = (I1 * I1 * R * DC) + (I2 * I2 * R * DC)
I1 = current at turn on
I2 = current at turn off
R = snubber resistor (7.5 ohm)
DC = duty cycle of current flow
how can i determine I1, I2 and DC ???

[bharbour]

I looked at the data sheet on the ST web site and it looks like you only need the first and second equations to calculate the RC values.

In the second equation, you are asking about dt/dv, The data sheet calls that dv/dt, which is the risetime of the output voltage in volts per second (or microSecond). The data sheet says that 200 volts per microsecond is "generally used" for dv/dt.

I did not see the snubber power dissipation equation in the data sheet. Duty Cycle is usually calculated as On time / PWM period, but that does not make sense here. Where did this equation come from?

The purpose of the snubber is to control the rate of voltage change on the motor power. The main reasons for doing this that I know of are to protect the driver chip and to prevent or minimize radiated radio interference. If you are seeing big voltage spikes around the turn on or off, I would put a large electrolytic cap (1000's of uF) and a small ceramic cap (.1 or .001uF) across the power supply pins of the L6203 chip, physically very close to the chip.

Bob

[xinzc28]

all the equations are came from ST application note for snubber on h-bridge.
http://us.st.com/stonline/books/pdf/docs/1703.pdf

[bharbour]

I looked at that app note and for the second equation, it looks like dt/dv is correct which in this case is 1 microsecond / 200 Volts, though the app note uses 150nS/50V.

For the third equation, the snubber conducts twice per PWM cycle, once at turn on and once at turn off. I think that the two currents are calculated for turn on and turn off transients and calculated in the equation 4 above this one in the app note. It looks to me like the two Duty Cycle numbers would be the period of the snubber conduction divided by the complete period of the PWM signal. This power number would be useful for calculating the resistor power rating.

Good Luck,
Bob