AN280

Source of voltage transients

Doc ID 1703 Rev 3

5/15

2

Source of voltage transients

To protect against the overvoltage that may occur as a result of the inductive property of the

load, voltage clamps are normally employed to limit the voltage across the output devices. In

bridge applications these clamps are normally a diode bridge that clamps the voltage to one

diode drop above supply and one diode drop below ground. However, if the diode switches

slower than the transistor, there is a short time where neither the transistor nor the diode is

conducting and the voltage rise is limited only by the capacitance on the node. The result is

that a voltage overshoot occurs during the time before the diode turns-on. When the bridge

is built with DMOS power transistors, the intrinsic body diode is often used as the clamp.

This is true for the L6201, L6202 and L6203. As can be seen in the Figure 2, the turn-off

time of the DMOS device in the L6203 is in the range of 25 to 50 ns while the turn-on time of

the intrinsic drain to source diode is in the range of 150 ns. This difference in switching time

is characteristic of many DMOS devices.

Figure 2.

Output switching waveform for the L6203

The second main factor contributing to the transients is the parasitic inductance in the wiring

or printed circuit board layout. Figure 3 shows the parasitic inductances in the DC motor

application. When the current flowing in these parasitic inductances is rapidly switched, the

inductive property of the wire causes a voltage transient. When large currents are rapidly

switched, as with DMOS transistors, large voltage transients can be induced across even

small parasitic inductances. For an inductive load driven by an H-bridge the change of

current in the power supply lead is equal to twice the load current when the bridge is

switched off or the bridge is switched from one diagonal pair of transistors being on the other

pair. The time that it takes to switch the current is essentially the turn-off time of the output

device. In this case the resulting voltage across the inductance is given by the equation:

Equation 1

AM15169v1

VL

di

dt

-----

L2

Il

Toff

-----------

⋅⋅

=

⋅

=