Design Idea:
35 Watt Isolated DC/DC Converter
Replaces Modules at Half the Cost

by Robert Sheehan

Introduction

The cost of Isolated DC/DC converter modules can be a big part of your system budget. At the 35 watt power level, the price can be greater than one dollar per watt—in some cases as high as two dollars per watt. Demonstration circuit DC227A provides an on-board solution for around fifty cents per watt.

Features

DC227A-A is designed for 500VDC isolation and lowest cost; it uses a standard Coiltronics VERSA-PAC^TM transformer and a Pulse Engineering inductor for the output filter. DC227A-B has 1500V isolation and uses a semicustom transformer, also from Coiltronics. DC227A-C has 500VDC isolation and achieves the highest efficiency using a Panasonic type PCC-S1 inductor for the output filter. The efficiency curves in Figures 2–5 are quite competitive, reaching 85% for the DC227A-C with a 5V output. The efficiency at 3.3V out is somewhat lower, due to the fixed losses of the output rectifier.

Circuit Description

This single-ended forward converter operates at a nominal switching frequency of 200kHz. Referring to the schematic in Figure 6, pulse width modulation is controlled by U1, an LT1247 current mode PWM controller. Transformer T2 and optocoupler Q7 provide galvanic isolation. C2 is a local bypass cap to reduce common mode–induced current.

To achieve fast start-up time, a hysteretic buck regulator is used for the bias supply power. U2, an LT1431 shunt voltage regulator, provides control for this function, with Q1 acting as the switch element; L2 and C21 provide output filtering. Q2 and Q4 protect the circuit during a hot plug, making this a very robust design; it is also impervious to output short circuits. The input surge voltage is limited to 80V by the rating of Q1–Q4.

The main switching power path through T2 comprises L1 and C18 as the input filter, Q6 as the primary switch, D7 as the secondary rectifier and L3 and C14, C16, C17 and C20 as the secondary filter. Transient voltage suppressor D8 is used to protect Schottky diode D7 during large-signal transient conditions. Power is transferred during the on cycle of Q6 and integrated by the output filter, just as in a buck regulator. The input filter component values for L1 and C18 are optimal and should not be changed without careful evaluation. C19 damps the input filter and will provide adequate stability for large values of input inductance. See LTC Application Note 19 for a discussion of input filter stability analysis.

Output voltage feedback is controlled using U3, another LT1431 shunt voltage regulator, as an error amplifier. In the event of a fault on the output power or sense lines, Z1/Q5 will override U3 and provide overvoltage protection. R10 and R21 are sized to handle any overvoltage condition. During an output short-circuit condition, the LT1247 is able to decrease the on time of Q6 to less than 200ns. This results in good control of the output short-circuit current, keeping power dissipation to a manageable level. The demonstration circuit uses surface mount devices for Q6 and D7. For elevated temperature operation at the full rated load, TO-220 devices can by mounted on a standard half-brick heat sink. For –48V inputs that require hot swap capability, the LT1640H negative voltage HotSwap controller provides a seamless interface. Demonstration circuit DC223A-B using the LT1640HCS8 is the recommended solution for use with the DC227A.

Figure 6. 35W isolated DC/DC converter schematic diagram