ISL78223 Datasheet, PDF(17/20 Page) Intersil Corporation – ZVS Full-Bridge PWM Controller with Adjustable Synchronous Rectifier Control

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ISL78223 Datasheet, PDF (17/20 Pages) Intersil Corporation – ZVS Full-Bridge PWM Controller with Adjustable Synchronous Rectifier Control

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ISL78223

When the upper switches toggle, the primary current that was

flowing through UL must find an alternate path. It

charges/discharges the parasitic capacitance of switches UL and

LL until the body diode of LL is forward biased. If RESDEL is set

properly, switch LL will be turned on at this time. The output

inductor does not assist this transition. It is purely a resonant

transition driven by the leakage inductance.

VIN+

VOUT+

RTN

VIN-

FIGURE 18. UPPER SWITCH TOGGLE AND RESONANT TRANSITION

The second power transfer period commences when switch LL

closes. With switches UR and LL on, the primary and secondary

currents flow as indicated in Figure 19.

VIN+

VOUT+

RTN

VIN-

FIGURE 19. UR - LL POWER TRANSFER CYCLE

The UR - LL power transfer period terminates when switch LL

turns off as determined by the PWM. The current flowing in the

primary must find an alternate path. The current flows into the

parasitic switch capacitance which charges the node to VIN and

then forward biases the body diode of upper switch UL. As before,

the output inductor current assists in this transition. The primary

leakage inductance, LL, maintains the current, which now

circulates around the path of switch UR, the transformer primary,

and switch UL. When switch LL opens, the output inductor current

free-wheels predominantly through diode D1. Diode D2 may

actually conduct very little or none of the free-wheeling current,

depending on circuit parasitics. This condition persists through

the remainder of the half-cycle.

VIN+

VOUT+

RTN

VIN-

FIGURE 20. UR - UL FREE-WHEELING PERIOD

When the upper switches toggle, the primary current that was

flowing through UR must find an alternate path. It

charges/discharges the parasitic capacitance of switches UR

and LR until the body diode of LR is forward biased. If RESDEL is

set properly, switch LR will be turned on at this time.

VIN+

VOUT+

RTN

VIN-

FIGURE 21. UPPER SWITCH TOGGLE AND RESONANT TRANSITION

The first power transfer period commences when switch LR

closes and the cycle repeats. The ZVS transition requires that the

leakage inductance has sufficient energy stored to fully charge

the parasitic capacitances. Since the energy stored is

proportional to the square of the current (1/2 LLIP2), the ZVS

resonant transition is load dependent. If the leakage inductance

is not able to store sufficient energy for ZVS, a discrete inductor

may be added in series with the transformer primary.

Synchronous Rectifier Outputs and Control

The ISL78223 provides double-ended PWM outputs, OUTLL and

OUTLR, and synchronous rectifier (SR) outputs, OUTLLN and

OUTLRN. The SR outputs are the complements of the PWM

outputs. It should be noted that the complemented outputs are

used in conjunction with the opposite PWM output, i.e., OUTLL

and OUTLRN are paired together and OUTLR and OUTLLN are

paired together.

FN7936.1

January 2, 2013

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