When the switch is OFF :

Eoff

= Vs ⋅ Ipk ⋅

tcom

2

The energy lost by commutation in a chopping period, given by Eon + Eoff, is :

Ecom

= Vs ⋅ Iload ⋅ tcom

The energy lost by commutation during the Tload time is given by :

Ecom

= Vs ⋅ Iload ⋅ tcom ⋅ Tload ⋅ fchop

Quiescent Power Dissipation, Pq.

The power dissipation due to the quiescent current is Pq = Vs

⋅ Iq , in which Iq is the quiescent current

at the chopping frequency, fchop = 1/Tchop.

Total Power Dissipation.

Let’s evaluate the power dissipation of the device driving a three phase brushless motor in chopping cur-

rent control. In the driving sequence only one upper DMOS and a lower one are on at the same time

(see fig. 9 and 10). The total power dissipation is given by :

Ptot

=

2

⋅ (Erise + Efall + Eload + Ecom)

T

+ Pq

Figure 11 shows the total power dissipation, Pd, of the L6234 driving a three phase brushless motor in

input chopping current control at different chopping frequency.

EVALUATION BOARD.

The L6234 Power SO20 board has been realized to evaluate the device driving, in closed loop control, a

three phase brushless motor with open collector Hall effect sensors.

ILOAD

I1B

I1A

I2B

half bridge 1

half bridge 2

OUT1

OUT2

I1B

I2B

I1A

ILOAD

Vs

ON/OFF

OFF/ON

ON

OFF

_

PHASE 12

CHOPPING INPUT

IOFF

Figure 9. Input chopping current circulation.

AN1088 APPLICATION NOTE

5/14