Switch mode power supply (SMPS) are power supply units that consists of a rectifier and a DC-DC converter. The SMPS draws power from an AC mains supply at a given frequency and voltage, around 60Hz and 240V for single phase units. The SMPS converts the AC voltage to DC and regulates it to the required level by stepping the voltage down or up. Therefore, SMPS can be used as a normal AC/DC or DC/C converter (Jain par. 2). This is a paper on the operation of Switch Mode Power Supply.
The rectifier in an SMPS receives AC from the mains and converts it do DC. The diode bridge in the rectifier offers full wave rectification to the AC. A filter circuit that consists of a capacitor is used to smoothen the rectified current before feeding it to the converter. A high frequency transformer sits between the rectifier and the converter. Also, the DC/DC converter has a high frequency switching circuit which regulates the DC output of the whole unit according to the required voltage (Jain par. 8). A voltage feedback from the load is relayed from the load to the circuit though an opto isolator. The regulator then outputs a voltage in line with the received signal. This type of voltage regulation is normally used in battery based systems, such as battery chargers. Voltage regulation in SMPS systems is achieved through cut-off or saturation mode (Coates par. 3).
Figure 1: Schematic diagram of Switched Mode Power Supply Unit (SMPS)
The voltage supplied to the load can be given by the equation:
Where Vout is the SMPS voltage output, Vin the voltage input at the switching circuit, D the duty ratio and Ns and Np are the primary and secondary windings of the isolating transformer. The above SMPS has a switching frequency of 16.2Hz, a Ns/Np ratio of of 2, and an output of 12V. The accompanying SMPS parameters can be determined as follows.
The input voltage:
The duty ratio D:
The waveforms for the various voltages are as follows:
Figure 2: The wave form of the input AC voltage with
The input voltage has a period of 0.01675 seconds derived from its frequency of 60Hz. The power has a sinusoidal wave form with a root mean square value of 45V and peak voltage of 63.64V.
Figure 3: The rectifier output waveform
The rectifier converts AC to DC, which has a single polarity. The filter capacitor smoothens out the troughs of the rectifier output so that the voltage is almost equal to the peak voltage.
Figure 4: The voltage across diode D1
The voltage across diode D1 is a half square wave due to the half wave rectification action of a single diode.
Figure 5: Voltage across the load
When the switching circuit S is on at time Ton, the output voltage Vs is equal to the input voltage from the transformer. When the switch S is turned off at Toff, the supply voltage is off and the load current flows through the freewheeling diode D2 and the output voltage is Vo. The load current is continuous, during Ton, the load current rises to peak value but decays during Toff ( Circuits Today par. 7).
The transformer in the switching section works even when DC is supplied to the converter because the switching circuit converts the DC from the rectifier to high frequency square wave AC (Coates par. 8). This AC current is then driven through the transformer after which it’s converted back to regulated DC output. The square waveforms helps in regulating the output voltage through a method called pulse width modulation. This method is efficient than in power supplies with linear regulation (Jain par.7).
Diode D1 converts the high frequency AC from the secondary windings of the transformer back to DC through half wave rectification. The voltage is then filtered by the inductor and the capacitor to a smooth DC output.
Circuits Today. Choppers-A general Introduction, n.d. Web. 23 July 2015.
Jain, Preeti. “SMPS (Switched Mode Power Supply)”. Engineer Garage, 2012. Web. 23 July 2015.
Coates, Eric “Switched Mode Power Supplies”. Learnabout Electronics, n.d. Web. 23 July 2015.