A Product Line of Diodes Incorporated AN59 Designing with Shunt Regulators - Series regulation Peter Abiodun A. Bode, Snr. Applications Engineer, Diodes Incorporated Introduction Series regulators are versatile means of supplying power to many types of electronic circuits. Although both fixed and adjustable regulators are readily available off the shelf at competitive prices, there are instances when it may be desirable or necessary to design a discrete solution. Such reasons might be due to requirement for a more accurate source than is commercially available - i.e. a precision regulator. These might be for instrumentation, calibration or environmental reasons. Low noise or level of required power might be other possible reasons for seeking a discrete solution. The Zetex precision references can make implementation of these discrete solutions relatively simple. This document details the design considerations of the series regulator. Basic Series Regulator ZXTN25020CFH Vin I R3 Q1 R3 IKA REF1 IB IL Vout IR R1 ⎞ ⎛ VO UT = VREF ⎜ 1 + ⎟ R 2⎠ ⎝ R1 VREF C1 R3 = (I R2 VIN (min) − (VOUT + VBE (ma x) ) B (ma x) IR 3 + I KA (min) ) ≤ IR 3 ≤ I KA (ma x) GND Figure 1 Basic series regulator using a reference A series regulator has the advantage that the series element (Q1) has a different voltage across it from that across the load. This voltage can be arranged to be very low in comparison leading to a much lower power dissipation in the series element, Q1, than is being delivered to the load. In addition, There is little or no power dissipation in Q1 if there is no load (i.e. when IL = 0) other than the very small current due to IR. These are the reasons why it is a better method for medium/ high power applications than the shunt regulator alone. It is an irony and a practical benefit that the most important basic building block of a series regulator is a shunt regulator which can be readily identified in Figure 1. The series regulator is, at most, only as good as the quality of this shunt regulator which, in its crudest form, could be a zener diode. The function of the shunt regulator is not to power the load but to drive the transistor which powers the load. As long as Q1 is suitably sized, this simple circuit can be configured to supply very high power and current running into several 10's of amperes and hundreds of Watts. Q1 could be a single device as shown or it could be a Darlington pair or even several transistors in parallel as the need may be. Accuracy The accuracy of the circuit is affected only by three components: the tolerance of the reference and that of resistors R1 and R2. See Appendix in AN57 for further information on this. Issue 2 - June 2010 © Diodes Incorporated 1 www.diodes.com AN59 Calculated Example 1 Requirement Supply Voltage: Output voltage: Load current: 12V to 15V 10V ±1% 100mA Assume the use of TLV431. Discussion The ZXTN25020CFH transistor is used in this example because it offers a high forward gain (180 - 500) and a fairly high power handling capability (1.25W) for a device in a SOT23 package. The equations for determining R1, R2 and accuracy are the same as in all reference applications. The objective here is to determine R1, R2 and R3 and check that the transistor Q1 can handle any resultant power dissipation. Solution R1 and R2 are chosen to be high enough in value that significant power is not wasted. On the other hand they need to be low enough that regulation accuracy and stability are achieved. Either R1 or R2 can be arbitrarily fixed and the other calculated from R1 ⎞ ⎛ VOUT = VREF ⎜1 + Equation 1 ⎟ R 2⎠ ⎝ AN58 showed the benefit of choosing a value of R1 in the region of 100kohm. Assume R1 is 100k and rearrange Equation 1 to obtain R1 R2 = ⎛ VOUT ⎞ ⎜⎜ − 1⎟⎟ ⎝ VREF ⎠ R2 = 100k ⎛ 10 ⎞ − 1⎟ ⎜ ⎝ 1.24 ⎠ = 14.15k Or R2 = 14.2k to the nearest E192 value and within 0.35%. Determine maximum required base current, IB(max) I I B(max) = OUT (max) h FE (min) +1 = Hence, 100mA 181 IB(max) = 552.5µA This is the maximum base current required by the transistor. R3 needs to be able to supply this plus, at least, the minimum cathode current for the TLV431 which is 100µA. Issue 2 - June 2010 © Diodes Incorporated 2 www.diodes.com AN59 Therefore, IR3(min) = 652.5µA R3 = R3 = Or VIN (min) − (VOUT + VBE (max) ) I R 3(min) 12 − (10 + 0.9) 652.5 μA = 1.68 k⍀ 1.6 k⍀ to the nearest lower E24 value. Off-load and at maximum input, all of IR3 will flow into the TLV431 and it is necessary to check that this current will not be excessive. Hence, VIN (max) − (VOUT + VBE (min) ) I R 3(max) = R3 15 − 10.6 1600 = 2.75mA less than 15mA as required. The last thing to check is that the transistor is suitably power-rated for this application. Hence, ⎛ h ⎞ FE (max) ⎟IOUT PQ1(max) = (VIN (max) − VOUT ) ⋅ ⎜ ⎜h ⎟ ⎝ FE (max) + 1 ⎠ = (15 − 10) ⋅ 0.998 ⋅ 0.1 = 0.5W This is comfortably within the capability of the ZXTN25020CFH when suitably mounted. Accuracy It is necessary to determine the required component tolerances for meeting the specified accuracy. Since the design calls for an accuracy of ±1%, it follows that the TLV431 used must be the 0.5% tolerance part. The resistors' tolerance can then be calculated as follows (see AN57 Shunt Regulators for more information): This is the error caused ⎛ R1 ⎞ First determine α RD α RD = ⎜ ⎟(α R1 − α R 2 ) by using preferred ⎝ R1 + R 2 ⎠ resistors as opposed to calculated values. 100 k ⎛ ⎞ ⎟(0 − 0.35) ⎝ 100 k + 14.15k ⎠ α RD = ⎜ α RD = −0.31% R1, R2 tolerance, ⎡⎛ α VOUT − (α TLV 431 + α RD ) ⎞⎛ R1 + R 2 ⎞⎤ ⎟⎜ ⎟⎥ 2 ⎠⎝ R1 ⎠⎦ ⎣⎝ α R = ± ⎢⎜ ⎡⎛ 1 − (0.5 − 0.31) ⎞⎛ 114.2 ⎞⎤ ⎟⎜ ⎟⎥ 2 ⎠⎝ 100 ⎠⎦ ⎣⎝ α R = ± ⎢⎜ α R = ± 0.463% Issue 2 - June 2010 © Diodes Incorporated 3 www.diodes.com AN59 Summary Using a TLV431B, R1 = 100k, R2 = 14.2k (both 0.463% or better) and R3 = 1.6k will satisfy the requirement. Series Regulator with Current Limit I LIM Rs ZXTN25020CFH Vin I R3 IB R3 Q1 I OUT VRE F REF2 I CONT VREF REF1 R1 Vout R1 ⎞ ⎛ VOUT = VREF ⎜1 + ⎟ R 2⎠ ⎝ R3 = (I C1 R2 VIN − (VOUT + VREF + VBE ) IR 3 B (max) RS = + I KA(min) ) ≤ I R 3 ≤ I KA(max) VREF I LIM GND Figure 2 Series regulator with current limit The circuit in Figure 1 has no current limit. If a short circuit were to be applied to the output, the resultant current, ISC, that would flow can potentially be ⎛ VIN (max) − VBE ) ⎞ ⎟⎟ ISC = (hFE (max) + 1)⎜⎜ R3 ⎝ ⎠ ⎛ 15 − 0.9 ⎞ = (501)⎜ ⎟ ⎝ 1600 ⎠ ISC = 4.4A The ZXTN25020CFH, with its IC(cont) rating of 4.5A could handle this current. A much more serious issue is that all of the supply voltage now appears across Q1 at the same time. At 15V this amounts to a power dissipation of 66W. According to the Pulse Power Dissipation chart in the ZXTN25020CFH datasheet, the transistor will fail in less than 0.3ms if subjected to this level of power. Figure 2 adds current limit to the series regulator in Figure 1 using a second reference (REF2), also a TLV431. For currents below the limit, the circuit works normally supplying the required load current at the design voltage. However, should attempts be made to exceed the design current set by REF2, the device begins to shunt current away from the base of Q1. This begins to reduce the output voltage and thus ensuring that the output current is clamped at the design value. Subject only to Q1's ability to withstand the resulting power dissipation, the circuit can withstand either a brief or indefinite short circuit. Calculated Example 2 Requirement Add a 105mA ±5% current limit to Calculated Example 1 (Figure 1 Basic series regulator). Solution RS = = or Issue 2 - June 2010 © Diodes Incorporated VREF I LIM 1.24 0.105 = 11.81 RS = 11.8 ⍀ To nearest E48 value. 4 www.diodes.com AN59 Using a 1% device (TLV431A), an RS value of 11.8⍀ 1% makes the worst case cumulative error 2%, within ±5% as required. Determination of actual short circuit current When the current limit circuit is operative, the output current consists of two components as shown below. ILIM remains constant, ICONT varies continuously with the level of overload until reaching a maximum at full short circuit i.e. when VOUT = 0. Hence IOUT (max) = I LIM + ICONT ⎛ VIN (max) − (VREF + VBE ) ⎞ I = I LIM + ⎜⎜ − LIM ⎟⎟ R3 hFE + 1⎠ ⎝ ⎛ h ⎞ VIN (max) − (VREF + VBE ) = I LIM ⎜⎜ FE ⎟⎟ + R3 ⎝ hFE + 1⎠ ≈ I LIM + Therefore, VIN (max) − (VREF + VBE ) IOUT (max) = since hFE >> 1 R3 1.24 15 − (1.24 + 0.6) + 11.8 1600 IOUT(max) = 113.3mA This represents the maximum short circuit current, it is not the current seen by transistor Q1 which is given by ⎛ h IC = I LIM ⎜⎜ FE ⎝ 1 + hFE ⎞ ⎟⎟ ⎠ if hFE >> 1 ≈ I LIM Therefore IC = 105mA This means that a direct short circuit would not immediately result in a failure. It is still however necessary to estimate how long the circuit could withstand such an overload condition for as follows. Overload duration With a short circuit, the current will be limited to a maximum of 107mA (105 +2%). Worst case voltage across Q1 will be 13.76V (i.e. 15V - 1.24V). Therefore, Q1's dissipation will be 1.47W. Referring to the Pulse Power Dissipation chart in the ZXTN25020CFH datasheet, it can be seen that Q1 will withstand this condition for between 10s and 100s. This is if there is a direct short circuit of the output voltage. If there is only a partial overload, the situation will be far less severe. There are a number of steps that can be taken if an indefinite short circuit handling capability is required. The simplest action would be to use a slightly bigger transistor for Q1. For example, the ZXTN2005G is a transistor in a SOT223 package and will dissipate up to 3W continuously when suitably mounted. Another method that could be used is to apply a re-entrant or "fold-back" current limiting rather than the simple current limiting above. This is a method which adjusts the over-load current limit according to the value of the output voltage such that, by the time the output voltage drops to zero - i.e. a short circuit, the current limit has dropped to a very small value, typically 5% or less of the full load current. This is a more complex solution and is outside the scope of this document. Issue 2 - June 2010 © Diodes Incorporated 5 www.diodes.com AN59 Conclusion Precision series regulators can be implemented using references. These allow the user to have more control of both the qualitative (e.g. accuracy) and quantitative (e.g. output voltage, current limit or power delivery) to suit the application. Recommended further reading AN58 - Designing with Shunt Regulators - Shunt Regulation AN60 - Designing with Shunt Regulators - Fixed Regulators and Opto-Isolation AN61 - Designing with Shunt Regulators - Extending the operating voltage range AN62 - Designing with Shunt Regulators - Other Applications AN63 - Designing with Shunt Regulators - ZXRE060 Low Voltage Regulator Issue 2 - June 2010 © Diodes Incorporated 6 www.diodes.com AN59 Issue 2 - June 2010 © Diodes Incorporated 7 www.diodes.com AN59 IMPORTANT NOTICE DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION). Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or other changes without further notice to this document and any product described herein. Diodes Incorporated does not assume any liability arising out of the application or use of this document or any product described herein; neither does Diodes Incorporated convey any license under its patent or trademark rights, nor the rights of others. Any Customer or user of this document or products described herein in such applications shall assume all risks of such use and will agree to hold Diodes Incorporated and all the companies whose products are represented on Diodes Incorporated website, harmless against all damages. Diodes Incorporated does not warrant or accept any liability whatsoever in respect of any products purchased through unauthorized sales channel. Should Customers purchase or use Diodes Incorporated products for any unintended or unauthorized application, Customers shall indemnify and hold Diodes Incorporated and its representatives harmless against all claims, damages, expenses, and attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized application. Products described herein may be covered by one or more United States, international or foreign patents pending. Product names and markings noted herein may also be covered by one or more United States, international or foreign trademarks. LIFE SUPPORT Diodes Incorporated products are specifically not authorized for use as critical components in life support devices or systems without the express written approval of the Chief Executive Officer of Diodes Incorporated. As used herein: A. Life support devices or systems are devices or systems which: 1. are intended to implant into the body, or 2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in significant injury to the user. B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or to affect its safety or effectiveness. Customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support devices or systems, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of Diodes Incorporated products in such safety-critical, life support devices or systems, notwithstanding any devices- or systems-related information or support that may be provided by Diodes Incorporated. Further, Customers must fully indemnify Diodes Incorporated and its representatives against any damages arising out of the use of Diodes Incorporated products in such safety-critical, life support devices or systems. Copyright © 2010, Diodes Incorporated www.diodes.com Issue 2 - June 2010 © Diodes Incorporated 8 www.diodes.com