SCR for Half Wafe Rectifier

Figure 1. Half waveSCR rectifier circuit

In my oldest article SCR characteristic, I already mention that SCR is used for ac phase control applications, both for rectifier and dimmer. In this article we work with half wave rectifier. Mostly, the ac to dc converter or rectifier circuit is built from power diode. SCR can be used to replaced power diode for a variable dc output voltage applications; so it called controlled rectifier.

The simplest controlled rectifier uses a single SCR, to produce variable voltage d.c. from fixed voltage a.c. mains. The circuit arrangement is shown in figure 1.

Operating principles for Resistive load

In the above arrangement, we have:

vs = Vm sin ωt = 0.707 V sin ωt = 0.707V


where Vm and V are the maximum and rms (effective) value of the supply voltage, respectively.


We note that from oldest article, the SCR is turned-on in the positive half-cycle (forward biased), some-time after supply voltage increase from zero, by the application of a positive gate pulse with delay angle a. For the negative half-cycle, the SCR is reverse biased and cannot switch on. In the other hand, the negative cycle will be blocked by SCR.
If we increase the delay angle (also called firing angle), the smaller average load voltage will be applied to the load. Otherwise, the higher average output voltage will be feed by the load if we decrease a. Note that: The average load voltage (Vd) from SCR half wave rectifier is found by calculating the area under the voltage curve then dividing by the length of the base. For any delay angle a, the average load voltage is represented by

Vd = (Vm/π) (1+ cos a)

To make a clear understanding of SCR half wave rectifier, please see figure 2. In this figure, we show the SCR voltage waveforms for two different delay angles.






Figure 2. dc output voltage which form by SCR half wave rectifier for two different firing (1)


Case example:(taken from ref 1)

A thyristor half-wave controlled converter has a supply voltage of 220V at 50Hz and a load resistance of 100 Ω. What are the average values of load voltage and current when the firing delay angle is (a) 30°(b) 90°?

(a) a =30°

Vd = (Vm/π) (1+ cos a)

Vd = (0.707.220/ π) (1+ cos 30°) = …. (please help me! Using your calculator)


(b) a =90°

Vd = (Vm/π) (1+ cos a)

Vd = (0.707.220/ π) (1+ cos 90°) = …. V


In summary, the average voltage or dc output voltage which form by SCR half wave rectifier is varied by adjusting the firing angle,a.

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References:

[1] Fewson. D(1998). Introduction to Power Electronics. Oxford University Press, Inc.
[2] Aripriharta, 2005-2012. Lecture Note. Power Electronics & Drives. Unpublished-for Education Only. State University of Malang, Faculty of Engineering, Departement of Electrical Engineering. (The original note, written in Bahasa Indonesia)

SCR parameters

In this article, we work with the most important parameters of SCR which always consider in practice.  These parameters are: gate current (I_G or sometimes written as I_GT), gate voltage (V_GT) forward break over voltage (V_BO), reverse break down voltage(V_DRM), rate current (mostly known as on-state current I_T{RMS)) and rate rms voltage (V). Used the SCR datasheet to find these values (pdf).

For example, let us consider the TN12, TS12 and TYNx12 Series from STMicroelectronics. From the SCR datasheets, I_T{RMS) parameters is safe to used for driving load current until 12A (rms). The TN12 SCR parameters is shown below. If this SCR is used for rectifier, the maksimum peak inverse voltage (PIV) is equal to 1000 V. It mean that, this SCR can block maksimum reverse voltage until 1000V. Then what about the SCR gate current? The positive low gate current I_GT varied from 0.2mA to 15 mA must injected by V_GT = 1.5V. So the gate resistance (R_G) may varied from 100 Ohm to 7.5 kOhm

Table 1. SCR parameters

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Symbol             Value                Unit

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I_T(RMS)                 12                     A

V_DRM/VRRM            600 to 1000       V

I_GT                     0.2 to 15           mA

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Source: STMicroelectronics

Note that: Either in sensitive (TS12) or standard (TN12 / TYN) gate triggering levels, the 12 A SCR series is suitable to fit all modes of control, found in applications such as overvoltage crowbar

protection, motor control circuits in power tools and kitchen aids, inrush current limiting circuits,

capacitive discharge ignition and voltage regulation circuits. Available in through-hole or surface-mount packages, they provide an optimized performance in a limited space area.

note: rms= root mean square or effective value .

SCR characteristics

SCR or Silicon Controlled Rectifier is one of power semiconductor devices which is popular for ac to dc converter, ac chopper, and ac phase control applications. As shown in figure 1, the SCR (sometimes called thyristor) is a three-junction P-N-P-N device, where P-N-P and N-P-N component transistors are connected in regenerative feedback mode. The SCR blocks voltage in both the forward and reverse directions; so that is a symmetric blocking. In the SCR characteristic curve or SCR volt-ampere characteristic or SCR static characteristic (figure 2), the SCR blocking voltage are: (i) forward break over voltage (V_BO), (ii) reverse break down voltage (V_BD).

Figure 1. SCR (left: symbol; right: characteristic)

A several techniques can be used to turn-on SCR. In this article, we only consider the normal method to turn-on SCR. If the anode (A) is positive, the SCR can be triggered into conduction by a short positive gate current pulse. But once SCR is conducting, the gate loses its control to turn off the device. The i-v characteristics of SCR indicate that at gate current I_G = 0, if forward voltage is applied on the device (mean that the anode is positive respect to the cathode), there will be a leakage current due to blocking of the middle junction. If the voltage exceeds its critical limit or equal to forward breakover voltage, V_BO), then SCR is ON-STATE. By increasing I_G, V_BO is reduced, and if I_G = I_G3, SCR behaves like a diode with the entire forward blocking region removed. SCR will turn on successfully if a minimum current, called a latching current (I_L), is maintained. During ON-STATE, if the I_G =0 and the anode current (I_A) falls below a critical limit, called the holding current (I_H), SCR reach its forward blocking state. With reverse voltage, the end P-N junctions of the device become reverse-biased and the i-v curve becomes similar to diode.  

In summary, SCR is turn-on by the positive gate current, and turn of naturally when the anode voltage drop to zero. And, each SCR has its own i-v characteristics. So, in practice we must consider the datasheet of SCR to get the important parameters of this device. There are, I_T(rms), V_BO , V_BD and I_G (see my other articles)

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