SANYO DENKI Technical Report No.6 Nov. 1998

General Theses

Floating Output DC-DC Converter Using Single

Winding Reactor and Its Applications

Hirohisa Yamazaki

1. Introduction

Networking based on a PC server is becoming increasingly popular and

uninterruptible power supplies (UPS) need to be made more compact, lightweight

and cheaper. The typical UPS now functions not only as a backup power supply

during a power failure, but also improves the power factor and cleans the

commercial power supply by absorbing the harmonics that are generated by

equipment acting as loads.

To achieve such targets, various rectifier circuits have been developed for UPSs.

Although Sanyo Denki has been marketing step-up chopper type and high frequency

insulation type products, various problems as discussed in Chapter 2 of this paper

make it difficult to use the products with 200 V input.

We report here on recent developments and a new product using a DC-DC

converter

(1)

that can generate an output that is floating (separated) from the input.

The new product is the 3 kVA UPS using the technology in the rectifier circuit. The

operating system of this DC-DC converter and UPS, and its characteristics are

described.

2. Requirements for Rectifier Circuit of Compact UPS

The typical load of a compact UPS is a personal computer. This means that the

compact UPS supplies AC power to the switching regulator of the power supply

circuit of the personal computer. Therefore, the output of the UPS includes various

harmonics having a high current crest value. At the same time, arresters and ceramic

varistors are used to protect the system from surges, and a high frequency filter is

inserted in the input circuit of the switching regulator to protect the system from

noise. The earth line of the system and the neutral line of the input/output AC

power line should have minimum potential difference so that the circuit components

do not operate on an abnormal potential with regard to the earth potential.

For that purpose, the input circuit and output circuit of a UPS should not be

separated, but the neutral lines of the input and output circuits should preferably be

connected directly.

The circuit in which full wave rectification and a full bridge inverter are combined

in the general purpose inverter is shown in

Fig. 1.

In this circuit, the DC voltage that exists in the middle of the output of the inverter

is super-imposed on the output voltage. This makes it difficult to connect the input

and output because of the potential difference between the neutral line of the input

circuit and output circuit. Therefore, the circuit operation becomes unstable with

regard to the earth potential and this can cause troubles such as burning of the

varistor of the load circuit or troubles due to noise.

In order to connect the neutral lines of the input and output circuits, the input

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could be isolated from the output.

The circuit configuration using an insulation transformer in the output circuit is

shown in Fig. 2. The circuit configuration of connecting an insulation transformer

in the input circuit, or inserting a DC-DC converter between the input and output,

has the same result, but insulation transformers are large, heavy and expensive. Use

of the DC-DC converter makes the circuit configuration complicated, and

manufacturing an insulation transformer for a large capacity inverter is difficult and

limits application of this circuit configuration to small capacity inverters.

A circuit configuration using a voltage-doubler rectifier + half-bridge inverter is

shown in

Fig. 3.

The rectifier circuit that is used in Fig. 3, is a high power factor converter circuit.

The DC voltage in the middle of this circuit becomes as follows when taking

fluctuation of the input voltage into consideration:

100V

1.15 2 = 325V

When this circuit is used for 200 V, double the voltage of the above figure becomes

necessary. This means that the semiconductors used in the latter stage of the

inverter must have a high withstand voltage, which reduces the efficiency.

Furthermore, it requires a larger air clearance and a longer creepage distance, which

hinder the design of a compact unit.

In addition, the mid-point voltage of capacity fluctuates in the half-bridge inverter

when an unbalanced load such as a half-wave rectifier load is connected.

Fluctuation of the mid-point voltage of capacity can create deviations of magnetism

and insufficient withstand voltage of the capacitor.

Each circuit system has its own merits and demerits as we have examined. We

have developed a floating output DC-DC converter (FCON) in order to solve these

problems for a UPS that operates on 200 V input and delivers output voltages of 3

kVA or more.

3. FCON Circuit

3.1 Operation of FCON

The FCON circuit that provides a floating output with respect to the input is shown

Fig. 4 (A).

The input energy is stored in the reactor L through the two semiconductor switches

Q1 and Q2. The stored energy is discharged through the two diodes D1 and D2. This

circuit operation is the same as that of a flyback converter or of a polarity-inverting

chopper circuit except that an insulating transformer is not necessary. The spike

voltage that is created normally in the polarity-inverting chopper circuit due to its

leakage inductance, is absorbed by Cdc and is suppressed.

The principle of obtaining a floating output using FCON is described below.

Fig. 4 (B) shows the energy storage mode in the reactor L.

When Q1 and Q2 are turned on, current flows into reactor L from the DC power

supply, and stores electromagnetic energy in L.

The capacity of the reactor that is required here is about 4% of the input

impedance, and is fairly small compared with the transformer.

During storage mode, the diodes D1 and D2 are reverse-biased by the reactor L

and the capacitor Cdc voltages, so that any current does not flow into the output

side that is separated from the input side.

When Q1 and Q2 are turned off, the system enters energy discharge mode as

shown in Fig. 4 (C). Energy that was stored in L during the storage mode is now

discharged into capacitor Cdc and the load side circuit through D1 and D2.

Because Q1 and Q2 are turned off during the discharge mode, the output side is

separated from the input side. This separation of the output from the input is

obtained by repeating the two modes of (B) and (C).

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