Capacitive Power Supply Design

Calculation of a capacitive power supply

Schematic uses single pulse rectification



For the function of a Zener diode: During the positive half-wave, D1 operates as a voltage-limiting component. The required output voltage can be acheived by adjusting the zerner diode value, in your case its 57V, Since zener is before D2 we need to consider voltage drop across D2 (0.7V), You should choose 57+0.7V zener diode D1.

During negative half cycle of input large amount of current flow through D1, it should be limited which can be done by R1.

R1 = Peak input voltage / max current through D1

Peak input voltage = 1.414 * 230V
Max current through D1 can be tacken from datasheet say 1A

R1 = 325.22 ohm.

Choose some nearest value say 330 Ohm

Since load current pass through R1,we should consider power dissipation.

Form factor of single pulse rectifier 2.2,
Actual load current 20mA *2.2 = 44mA
P = I*I*R
p = 44*44*330 = 0.6388W
This component will get heated up need to consider derating wrt temperature, lifetimne etc consider 2x

Power ratingof R1 = 1W

Voltage drop across resistor at full load

Vr1 = 0.6388/44mA = 14.5V

Now its time to calculate capacitor value

Capacitive reactance = 230V-23-14.5-57.7/44mA = 3063.63 Ohm

Capacitance C1 = 1/2Pi*f*XC = 1/(2*3.14*50*3063.63) = 1.03uF

C1 voltage rating should be higher than ac input, select x rated cap usually called as box cap or film cap.

D2 can be any diode 1N4001 commonly used.

C1 smothering of rectified AC.

Time period of 50Hz 20ms

During negative half cycle output should be tack care by C1, Hals cycle time period 10 mS

Ripple voltage can be considered based on application requirement and available size, economical factor, say 2% of output voltage ~ 1V

Load resistance = Voltage/ load current = 57/20mA = 2850 Ohm

C1 = -10ms / (2850 * ln(56/57)) ~ 200uF, nearest 220uF, 100V

Add some 0.1uF or lesser for noise elimination parallel to C1

Add fuse for protection

Note: This circuit doesn't provide any galvanic isolation

Thyristor Ratings

SCR Ratings

Subscript identification

First subscript

F - Forward bias

R - Reverse bias

T - ON state

Second subscript

T - Trigger

S- Surge or Non-repetitive value

R- Repetitive value

W- Working value

Voltage Ratings of SCR

1. Peak Working Forward-blocking Voltage VDWM

Forward blocking voltage across SCR in gate open condition. Beyond this value SCR will switch on and device will damage

2. Peak Repetitive Forward-blocking Voltage VDRM

It is the maximum transient voltage that the SCR can block during it’s the forward blocking state repeatedly or periodically.

3. Peak Non-repetitive or Surge Forward-blocking Voltage VDSM

Maximum non repetitive instantaneous voltage across SCR in forward blocking mode

4. Peak Working Reverse Voltage VRWM

Maximum Instantaneous value across SCR in reverse biased condition

5. Peak Repetitive Reverse Voltage VRRM

Maximum reverse transient voltage across SCR under reverse biased

6. Peak Non-repetitive or Surge Reverse Voltage VRSM

Maximum non repetitive instantaneous voltage across SCR in reverse blocking mode

7. ON-state Voltage VT

Voltage drop across the SCR at specific junction temperature

8. Gate Triggering Voltage VGT

Minimum voltgae required at gate for switching

9. Forward dv/dt Rating

Maximum rate of rise of anode voltage what will not turn ON the SCR

If dV/dt is more than specific value more charge will induce in J2 (PN junction inside SCR) and causes false trigger with our SCR.

Current Ratings of SCR

1. Average ON-state Current Rating ITAV

Maximum value of average current that can flow through SCR given junction temperature and rms current within limit

In case of phase controlled rectifier for given average current, rms current increases with decrease in conduction angle or increase in firing angle.

This lead to increase in more voltage drop across SCR and power dissipation, which need to be properly addressed for application.

2. RMS ON-state Current ITRMS

Maximum allowable SCR current given junction temperature within limit.

This parameter need to be properly addressed for wide duty ratio application.

3. I2t Rating

Determines thermal energy dissipation of device. Used majorly for selection of fuse rating. I2t rating of fuse should be less than I2t rating of SCR

This measures the thermal withstand capability of SCR before protective equipment clears the fault

4.  di/dt Rating

Maximum rate of rise of current that is allowable within junction temperature. This directly relate to rise time spread time for given gate current and voltage. Usually exopress in A/uS

5. Latching Current IL

Minimum ON state anode current to keep the device ON even gate current is removed

6. Holding Current IH

Minimum value of anode current below which SCR OFF if gate current is removed

7. Gate Current IG

Maximum and minimum value of gate current

Temperature Rating of SCR

Forward and reverse blocking state of SCR is determined by junction temperature Tj. This can change the current rating of SCR

1. Power Ratings of SCR

Product if average anode current and voltage drop across the SCR

2. Gate Power Dissipation PG

Product of gate current and voltage. Should be with in limit mentioned in datasheet. For high duty cycle application gate losses become significant

3. Turn ON and Turn OFF Time Ratings

Time between instant of gate signal applied to the rate of rise of anide current reached 90%