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AN1451 APPLICATION NOTE

Figure 4. Voltage at the two outputs at the beginning of the off-time.

2.3 Choosing the Bulk Capacitor

AC current capability must be greater than the r.m.s. value of the charge/discharge current. This current flows

pacitor depends on peak output current, output current ripple, switching frequency, duty-cycle and chopping

style. It also depends on power supply characteristics. A power supply with poor high frequency performances

(or long, inductive connections to the IC) will cause the bulk capacitor to be recharged slowly: the higher the

current control switching frequency, the higher the current ripple in the capacitor; r.m.s. current in the capacitor,

however, does not exceed the r.m.s. output current. Bulk capacitor value (

of voltage ripple on the capacitor itself and on the IC. In slow decay, neglecting the

ripple, and assuming that during the

the end of the

,

so the supply voltage ripple is:

,

the supply voltage to exceed the nominal voltage. This can be very dangerous if the nominal supply voltage is

close to the maximum recommended supply voltage (52V). In fast decay the supply voltage ripple is about:

,

always assuming that the power supply does not recharge the capacitor, and neglecting the output current ripple

and the dead-time. Usually (if C > 100 µF) the capacitance role is much less than the ESR, then supply voltage

ripple can be estimated as:

Out 2

Out 1

V

S

I

OUT

–

ESR

t

ON

C

---------

+





⋅

I

OUT

ESR

t

ON

C

---------

+





⋅

I

OUT

2 ESR

⋅

t

ON

t

OFF

+

C

----------------------------

+





⋅