P ROTECTIONS OF SMPS

In the design of a power supply it is prudent to provide protection circuitry to protect against extreme and abnormal operating conditions that will inevitably occur when the supply is in use. These can occur in the form of output short circuits and excessive loads or high voltage transients on the input supply line. Many of the components in a power supply are handling powers greatly in excess of their dissipation capability. Under fault conditions it is quite possible that they may start to dissipate this power, leading to their rapid failure. The power supply designer has no control over these faults and therefore must incorporate circuitry to accommodate them safely. This falls into five broad categories: (1) over-current protection; (2) over-voltage protection; (3) inrush protection; (4) device protection; (5) over temperature protection. [4], [42]

2.4.1 Over-Current Protection

In order to provide current limiting, some means of sensing over-current conditions must be provided. In an SMPS the commonest method of achieving current limiting is to control the switching activities of the switching transistors. Under fault conditions the transistors can be switched off. Cycle-by-cycle protection is a useful method of output current limiting in an SMPS.

The purpose of current limiting is two-fold: firstly to limit the dissipation in the power supply components to safe values and thereby prevent damage to them, and secondly to provide some protection to circuits and systems being powered by the

supply.

2.4.2 Over-Voltage Protection

Over-voltage protection must deal with three possible situations: (1) reverse voltage on output; (2) external over-voltage on output; (3) internally generated over-voltage. The first two situations are reasonably easily dealt with by placing

“catcher” diodes on the output. Fore reverse polarity protection, a normally reverse biased diode can be placed on the output. Normal over-voltage protection can be provided by a zener or avalanche diode whose voltage is in excess of the normal operating voltage of the power supply. Diodes for both types of protection must by amply rated to cope with the anticipated fault conditions.

It is important that a power supply does not give out an abnormally high voltage under fault conditions. If it did so it could easily damage the circuitry that it is powering. We can monitor the output voltage by an over-voltage control circuit. This is usually some form of comparator that is set to trigger under over-voltage conditions.

Its output is used to stop the switching activity thereby shutting down the power supply; in much the same way as was done for cycle-by-cycle current limiting.

2.4.3 Inrush Protection

In most SMPS designs it is desirable to introduce a certain delay during start-up, in order to avoid inrush current and output overshoots at turn-on. Circuits that are employed to perform this task are called soft-start circuits. In general they control the modulation circuitry to make the output to increase from zero to its operating value very “softly”.

Fig. 2.9 shows how a soft-start circuit may be implemented in a PWM control circuit. At time t = 0, when the power supply is just turned on, capacitor C is discharged and the error amplifier output is held to ground through diode D1, thus inhibiting the comparator output.

Fig. 2.9 A typical soft-start circuit used in a PWM control circuit aids the gradual increase of the PWM signal to its operating value.

At time t = 0⁺, the capacitor starts to charge through resistor R with a time constant determined by τ = RC toward the charging voltage V_SUPPLY. As capacitor C attains full charge, diode D1 is reverse biased, and therefore the output of the error amplifier is isolated from the soft-start network. The slow charge of capacitor C results in the gradual increase of the PWM waveform at the output of the comparator, and consequently a “soft start” of the switching element is initiated.

Diode D2 is used to bypass resistor R in order to discharge the capacitor C fast enough in case of system shutdown, thus initiating a new soft-start cycle even during very short interrupt periods. In some PWM control ICs, the resistor R has been substituted by an on-chip current generator, thus the only external element required to

implement the soft-start feature is the addition of capacitor C.

2.4.4 Device Protection

While dealing with protection it is probably worth mentioning voltage clamping.

Here an avalanche diode is placed across the two conduction terminals of the power switch transistor. The breakdown voltage of the diode is chosen so that it is below the breakdown voltage of the power switch transistor and greater than the supply. In normal operation the diode will not conduct, but it will serve to clip any high voltage transients that might be harmful to the transistor.

2.4.5 Over Temperature Protection

Although a power supply is protected by over-voltage and over-current protection circuits, it may undergo over heating condition because of high ambient temperature, improper heat sink design or malfunction of heat sink devices. The over temperature protection is needed to protect the power supply and all the devices and the circuits around the power supply under these conditions. Without over temperature protection, these abnormal conditions may cause fire and great damages.

The thermal sensors detect the temperature at the critical positions in the power supply. Once the temperature is higher than a preset safety margin, an over temperature signal will be issued to turn off the power switches. The heat energy will stop accumulation and the temperature may go down.