design ideas How to Design an Isolated, High Frequency, Push-Pull
DC/DC Converter
Dawson Huang A simple push-pull DC/DC converter with a fixed 50% duty cycle is often used as a low
noise transformer driver in communication systems, medical instruments and distributed
power supplies. This simple scheme provides no voltage regulation—requiring a low
dropout (LDO) post regulator—a combination that presents potentially serious problems.
First, any significant variation in the driver’s input voltage, along with the fixed 50% duty
cycle, can increase the differential voltage across the LDO, resulting in significant power
losses and high temperature rise in the LDO. Second, low switching frequency requires
relatively bulky transformers, sometimes occupying 30% to 50% of the converter space.
The LT3999 monolithic DC/DC push-pull
driver avoids these issues with two important features: duty cycle control and high
frequency operation: Duty cycle control allows compensation
for wide VIN variation—something standard fixed duty cycle transformer drivers cannot do—greatly reducing LDO
loss when facing a wide input range. High switching frequency, up to
1MHz , leads to smaller transformers and lower output ripple. wide input range, the other for a
compact high frequency transformer
driver with fixed input voltage. The LT3999 combines these two
features with high 36V input voltage
and 1A input current capabilities,
making it a high power and flexible
low noise push-pull converter IC. PUSH-PULL DC/DC CONVERTER
DESIGN FOR WIDE RANGING INPUT This article presents two step-bystep design procedures: One for a …