Introduction to regulated dc power supplies

Introduction to regulated dc power supplies  

Power supply is generated a fixed or controllable magnitude dc voltage from the available form of input voltage. In majority of the cases the required voltages are of magnitudes varying between -18 to +18 volts. Some equipment may need multiple output power supplies. For example, in a Personal Computer one may need 3.3 volt, ±5 volt and ±12-volt power supplies. The digital ICs may need 3.3 volt supply and the hard disk driver or the floppy driver may need ±5 and ±12 volts supplies.

There are two broad categories of power supplies: Linear regulated power supply and switched mode power supply (SMPS).

(1)  Linear regulated power supply.

                           Figure. (a) block diagram of Linear regulated power supply.

Fig. (a) shows the basic block for a linear power supply operating from an unregulated dc input. This kind of unregulated dc voltage is most often derived from the utility ac source. The utility ac voltage is first stepped down using a utility frequency transformer and then it is rectified using diode rectifier and filtered by placing a capacitor across the rectifier output. The voltage across the capacitor is still fairly unregulated and is load dependent, so adjust dc voltage at required level using linear regulator. The filter capacitor size is chosen to optimize the overall cost and volume. Here output voltage change as load change which is main drawback of linear power supply.

Advantage:

·         Simplicity, low noise levels and low cost, low complexity, Low EMI due to low frequency operation, cost less due to low number of component.

Disadvantage: -

·         In high power application requires a large transformer and other large components.

·         High heat loss occurs when regulating a high-power load.

·         Load change output voltage change.

      (2)  Switched-mode power supplies (smps)

 

                       Figure. (b) General block diagram of a switched-mode power supply.

Like a linear power supply, the switched mode power supply too converts the available unregulated ac or dc input voltage to a regulated dc output voltage. Switched Mode Power Supplies (SMPS) are replacing the linear power supplies due to their higher efficiency, better voltage regulation and capability to achieve an excellent power quality.

In SMPS circuit, the ac supply is first rectified and then filtered by a capacitor to produce an unregulated dc supply. The unregulated dc supply is fed directly to a high-speed power switching section which consists of electronic switches connected as a chopper or an inverter. The electronic switches are generally fast-switching power semiconductor devices such as MOSFETs or IGBTs. These power semiconductor devices are switched on and off to intermittently apply the unregulated dc voltage to the primary of the HF power transformer.

The switching pulses are normally fixed frequency (20 kHz to 200 kHz) and variable duty cycle. Semiconductor device output is rectified by a high-speed rectifier, and then smoothed by the output filter, which is either a capacitor or capacitor/inductor arrangement. The transfer of power has to be carried out with the lowest losses possible to maintain the best possible power efficiency. Thus, optimum design of the passive components, and selection of

the proper power semiconductors are critical. Here HF power transformer is step-up type due to high dc voltage outputs are required.

As the load increases, the voltage at the dc output of an SMPS decreases because voltage drops across the various circuit components (diodes, electronic switches, and the HF power transformer) increase. Voltage feedback loop is used in the SMPS to maintain the voltage at the dc output constant. The voltage feedback loop also compensates for fluctuations (ripple) in the rectified input voltage. Generally, most SMPS units operate using fixed-frequency, phase shift pulse-width modulation (PWM), where the duration of the on time (i.e., the duty cycle) is varied on a cycle-to-cycle basis to compensate for changes in the rectified input voltage and output load.

The voltage feedback loop subtracts the dc output voltage from the voltage command. Whenever the dc output voltage differs from the voltage command, an error signal is produced. The error signal, after some proportional and integral (PI) amplification, is used to adjust the duty cycle of the switching signals applied to the high-speed electronics witches in such a way as to correct the error on the dc output voltage.

 For instance, when the dc output voltage is lower than the voltage command, the error signal has a positive polarity and this makes the duty cycle increase by adjusting phase shift. Consequently, the rms value of the ac voltage at the HF power transformer increases, and thus, the dc output voltage increases to compensate the error.

When the dc output voltage is higher than the voltage command, the error signal has a negative polarity and this makes the duty cycle decrease by adjusting phase shift. Consequently, the rms value of the ac voltage at the HF power transformer decreases, and thus, the dc output voltage increases to compensate the error.

When the dc output voltage equals the voltage command, the error signal is zero, and the duty cycle is set to the exact value required to keep the system in equilibrium.

Advantages of switched-mode power supplies:

• Higher efficiency.
• Regulated output voltage with variations load.
• Small size and lighter.
• Flexible technology.

Disadvantages:

• Generates EMI
• Complex circuit design

SMPS VS Linear power supply.

SMPS	Linear power supply
Switching loss very less due to switching element (MOSFET, IGBT…etc)	More voltage drops across series element
Use high frequency transformer so size and weight reduce.	Use low frequency transformer so size and weight increase.
Due to high frequency, output voltage filtering circuit is cheaper.	Due to low frequency, output voltage filtering circuit bulkier.
To remove ripple, Filter element size is less.	To remove ripple, Filter element size is large.
High efficient.	Bulkier and less efficient.
Control of the SMPS circuit is much Complex than Linear power supply.	Control of the linear power supply circuit is much simpler than that of SMPS circuit
Electro- magnetic interference present due to high frequency switching.	Electro- magnetic interference absent due to low frequency switching.