Tri-M Systems HE104 Theory of Operation, Input power protection, Switching Regulator, +5VDC

23 June 2005

HE104MAN-V8 Manual

CHAPTER 3: THEORY OF OPERATION

3.1 Input power protection

Input power is connected to the screw terminal block, CN1, which is removable from the socket connector on the circuit board. A 10 ampere “pico” fuse F1 limits the current draw from the power source. A series of devices, (toroid coil L3, transorb D4 and filter capacitors, C8A,C8B and C8D) filters and clamps the input power.

Transorb D4 is a 5KVA, heavy-duty, transient suppressor. Transorb D4 provides “zener” type protection and has an avalanche voltage of 43V. It is electrically located before fuse F1 to prevent activation of the fuse during a “load dump” or large transient. Sustained voltages greater than the avalanche voltage must not be applied or transorb D4 will fail.

3.2 Switching Regulator, +5VDC

A switching regulator IC1, generates the +5VDC output, which operates in a “buck” mode synchronous switching regulator configuration. It does this by using inductor coil L1, upper mosfet Q4, lower mosfet Q5, schottky diode D8, input filter capacitors C8A, C8B, C8C, C8D and output filter capacitors C2, C3, C9A, C9B and C9C. Regulator IC1, is a current mode controller that adjusts the “switching cycle” by the sensed current, instead of directly by the output voltage sensing error amplifier, in regulator IC1. This further sets the current trip level. Operating frequency is set by capacitor C5.

A total of 15 amperes can be supplied to the connected +5VDC load and +12VDC regulator, the – 5VDC, -12VDC charge pumps and the invertors. The +5VDC power is available on the PC/104 expansion bus and screw terminal connector CN2.

At start-up, a low dropout 4.5V source located in regulator IC1, provides the operating voltage Vcc for the mosfets and control circuitry. After start-up, regulator IC1, is in a sleep mode whenever the output level is within the burst mode voltage limits. As soon as the +5VDC output drops below the burst low-level voltage, normal switching regulator operation begins and continues until the +5VDC output reaches the upper burst level voltage. Then regulator IC1 is put into sleep mode again. When low quiescent power mode is enabled, regulator IC1 burst mode operation will begin at approximately 1.5 ampere output. The output ripple when regulator IC1 is operating in burst mode, is 50millivolts.

The +5VDC regulator functions fully down to 6VDC input. Below 6VDC input, the +5VDC output will track the input and have a drop proportional to the resistive losses on the HE104. The HE104 operates up to 40VDC input.

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