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| New! ALL-FET low-noise SUPER-SHUNT regulator. |
Shunt regulators have been around a long time and recently have also been very popular in audio applications. Well-designed shunt regulators can outperform series regulators and even batteries in terms of transparency, dynamics, extended sound-stage, and super bass-performance. Resolution in low-level circuits like phono preamps is outstanding. Overall they get you closer to the live performance than any other regulator system.
Block schematic of a shunt regulator is shown in fig. 1. It consists of two parts: the Constant Current Source or CCS and the Shunt, which is made up of the error amplifier and the shunt element. The CCS is delivering a current equal: Iccs = Ish + Iload. The objective of the shunt regulator is to maintain a constant voltage Vreg across the load terminals, irrespective of the load current Iload. The shunt regulator accomplishes this by measuring Vreg through a voltage divider and comparing it to a reference voltage Vref. If the two voltages differ the error amp will force the shunt element to conduct more or less Ish thus maintaining a constant Vreg.
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 |  | | Fig. 1 - Block schematic for shunt regulator |
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The performance, i.e. how close the regulator is capable of maintaining a constant Vreg, depends mainly on the reference diode Vref and the error amp. The simplest shunt regs are based on a zener diode for Vref and a single bipolar transistor or a FET for the error amp (Ref. 1, 2). The high performance shunt regs all use a temperature compensated Zener diode or an IC reference diode for Vref and either an IC opamp or a discrete opamp for the error amp. In developing our shunt reg we compared and field tested three versions: a simple zener-bipolar transistor combination, an IC Vref and IC opamp combination and the IC Vref and discrete opamp combination. The last one won the competition in overall sonic performance.
The high-performance EB-906/255 SUPER-SHUNT regulator.
Fig 2 shows the schematic of the high performance Super Shunt regulator EB-906/255. The MOSFETs Q3 and Q4 form the Constant Current Source (CCS), delivering the current Iccs = Ish + Iload. Iccs is determined by the base-emitter voltage of the bipolar transistors Q1 and Q2 and is set by resistors R1 and R2: Iccs = 0.66/R1 (or R2). For 140mA total CCS current R1 (R2) = 0.66/140mA =4.7 Ohm. R1 (R2) =2.2 Ohm gives 300mA CCS current.
The shunt elements Q13 and Q14 are also MOSFETs and they are driven by discrete JFET/MOSFET opamps. The reference voltage is provided by IC reference diodes D3 and D4 and these are fed by JFET CCS D1/D2. The Vreg is compared to the reference voltage by resistor divider networks R13/R15/P1 and R14/R16/P2. P1 and P2 allow you to adjust the regulated voltage.
The CCS MOSFETs Q3/Q4 and the Shunt element MOSFETS Q13/Q14 have to be heatsinked by HS1/HS2 and HS3/HS4, which are SK 104 heatsinks. For maximum power dissipation by different sizes and application see Table 1.
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 | | | Fig. 2 - EB-906/255 High Performance Super Shunt Regulator |
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Heatsink
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Thermal res.
Rth (K/W)
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Max. Power dissipation
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CCS 4V Vin-Vreg
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Shunt element Vreg=24V
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SK 104 25mm:
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14K/W
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2.14W
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525mA
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89mA
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SK 104 38.1mm
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11K/W
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2.72W
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680mA
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113mA
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SK 104 50.8mm
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9K/W
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3.33W
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830mA
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138mA
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SK 104 63.5mm
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8K/W
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3.75W
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930mA
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156mA
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Table 1. SK-104 power dissipation and current capability for CCS and shunt element application at 30°C temperature rise over ambient.
The table shows the current capability for CCS and shunt element application, assuming that the voltage drop across the CCS is 4V and the output voltage Vreg is 24V. For other voltage drops and output voltages calculate the max. current the heatsink can support at 30°C temperature rise over the 25°C ambient. Note that these are theoretical values, assuming perfect air circulation around the heatsinks! (See note on ventilation under Setup Procedure.)
The EB-906/255 is offered in two versions: low load current up to about 100mA and high load current up to 300mA. The low current version is using the SK 104 25mm heatsinks for both CCS and shunt element. The CCS and the shunt element are TO-220 MOSFETs. The high current version uses the SK 104 50mm for both and the CCS and the shunt element are TO-3P MOSFETs.
The amount of current the shunt element is “shunting” can vary with the application, but most shunt reg manufacturers are suggesting min. 20% of the load current. If your load current is 100mA then the shunt element should shunt minimum 20mA, i.e. the total current the CCS is delivering should be 120mA. We recommend using as much as the shunt element can safely dissipate. This means a “waste” of power in the reg, but it is worth in terms of sound quality.
Let’s take an example. The shunt reg is used for our 320 phono preamp, which needs 200mA per channel at 24V. Using the high current version of the EB-906/255, the SK 104 50mm can dissipate 3.33W. This poses no problem as far as the CCS is concerned, with a 4V voltage drop it can supply >800mA. However, the shunt element can only shunt 138mA at 24V. In this case we recommend to adjust the total CCS current to 250mA (R1/R2 = 2.7 Ohm), meaning that the shunt element operates at 50mA, i.e. 25% of the load current.
Note that shunt regs should basically not be operated with Iload = 0 mA, i.e. open output, unless the total CCS current is less than the maximum shunt element current! If we take the above example, the shunt element will have to shunt all of the current from the CCS with open output, and with 24V and 250mA the power dissipation will be 6W!! Obviously this is NO GO! See Setup Procedure for proper operation of the shunt reg when you first switch it on!
One of the advantages of shunt regs that manufacturers are pointing out is that it is short circuit proof. Theoretically correct, however, you need to watch the power dissipation in the CCS this time. Assume you short the output of our shunt reg we described above. With 24V output and 4V across the CCS the Vin is 28V. If the CCS is delivering 250mA, then the power dissipation will be 7W, which is again NO GO. So watch your CCS and shunt element for max. power dissipation.
Typical specifications for EB-906/255
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Low power version
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High power version
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Input voltage range
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16-32
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16-32V
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Output voltage range
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12-28
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12-28V
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Min.
Diff. Vin/Vreg
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4V
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4V
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Max.
load current
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100mA
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300mA
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Max.
shunt current
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75mA
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150mA
(SK 104 63.5mm)
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Noise
5Hz-30kHz
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<10µ
(Typ. 5µV)
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<10µV (Typ. 5µV)
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Absolute max. Vin
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35V
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35V
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Absolute max. Vreg
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30V
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30V
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High Current Shunt Reg
Low Current Shunt Reg
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