VIPower high side switches are able to drive every kind of loads and actuators; goal of this note is a better knowledgment of the turn-off output clamp voltage: Vdemag.
When an High Side Drive turns off an inductive load, an internal circuit provides to dissipate through the power MOSFET the coil stored energy in such a way to eliminate the external free wheeling diode (see picture nr.1).
The current variation during turn-on and turn-off phases:
With an input signal the power MOSFET turns on and the current through the coil rises in such a way that the voltage across the coil is given by the well known expression:
(*) VL=L(dI/dt) and :
dI*L/VL =dt
Where L is the load inductance and dI/dt is the variation of the current. The load current has a typical slope of the picture nr.2.
During the turn off phase current decreases from the maximum value reached in turn-on state to zero; so a negative voltage VLOAD appears across the inductance.
The power MOSFET source (connected to the output pin of the H.S.D.) becomes more negative than the ground; between the drain (connected to the Vcc) and the source the voltage Vsustained is:
Vsustained =Vcc - VLoad
See picture nr.3: the stored coil energy Ecoil is given from the formula: Ecoil=0.5*L*I2
During turn off this Ecoil energy must flow through the power MOSFET; so the energy dissipated Edissipated on it is given by
the integral of the current for the voltage sustained;
using the above mentioned formula (*);
Thanks to an internal circuit, made by a series of zener diodes, the sustained voltage is CLAMPED at a value:
Vsustained=Vcc- Vdemag;
Where Vdemag is indicated in the datasheet beyond the electrical characteristics section.
For VN820, Vdemag=Vcc-41Volts (minimum value); in this case Vsustained=Vcc-Vdemag=Vcc-(Vcc-41)Volts=41Volts.
To find the energy dissipated on the power MOSFET stage is necessary to know the Vload which is given from Vload=-Vdemag.
The output voltage and the current slope are shown in picture nr.4: at turn off the current decreses linearly; the output voltage goes negative but is clamped at Vcc-41 Volts (for VN820), which is the Vdemag; on the diagram are indicated also Vload, Vsustained and Vdemag.
| VN820 can dissipate this energy? |
A calculation will clearly show what is the energy that VN820 must dissipate with the following data:
Coil inductance: 6mH
Vcc: 13Volts
Vdemag: Vcc-41Volts;
With the above values for L, Vcc and I, the max dissipated energy is above 500mJ, so our device is can safely drive the coil.
