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Projects: PTN78060W Switch Mode Voltage Regulator

Switch Mode Regulation Made Easy!

The PTN78060W is a very neat switch mode voltage regulater module that can be easily used to provide voltage regulated supplies of from 2.5 volts to say 10 volts at up to 3 Amps from a 12 to 15 volts source. It operates at efficiencies of around 95% so that there is no need for a cooling fin that is typically required for a comparable non-switching regulator.

Overall dimensions of the PTN78060W are only 25mm x 16mm x 9mm! (approx)

The PTN78060W data sheet describes how to use the device to provide fixed output voltages which are determined by the value of a single resistor. With a bit of inginuity, this device can also be used to provide variable output voltages. I have described how to do this below.

Important Note:
The PTN78060W operates at switching frequencies of around 500KHz so it is VERY IMPORTANT that the specified decoupling capacitors on the input and output sides of the device are installed correctly. A 2.2uF (that is micro Farads!) CERAMIC capacitor is REQUIRED on the input side, but don't let the description in the data sheet put you off! 2.2uF 25 Volt working ceramic capacitors are available, but you might need to search the Internet to find them. They are surface mount components which can be soldered across the input terminals using short pieces of copper wire. Keep the connections as short and as close to the input terminals of the module as is practically possible and all should be well.

The output capacitor can be a normal electrolytic capacitor, but refer to notes in the data sheet.

There's little point in discussing the normal operation of the PTN78060W any further here. This is done very well and clearly in the data sheet. I will describe my own tests and experiences which are aimed at developing ways of using the PTN78060W to provide variable output voltages and eventually to provide the basis of a temperature controlled Peltier system which could be used to cool a CCD astro camera or modified webcam.

Practical aspects of using PTN78060W:
Unlike simple DC series regulators, and even some basic switch mode regulators, the very high operating efficiency of the PTN7806W means that no cooling fins are needed!

My initial tests used the well known '3D rat's nest' method of building circuits. This demonstrated that:

1. Provided that the input and output capacitors are connected close to the terminals of the module, other connections could extend over several centimeters without problems.

2. I discovered by accident that the mounting pins of the PTN78060W can be removed simply being over enthusiastic with a soldering iron! Worrying at first, but actually a useful point. After removing these pins, short lengths of wire can be soldered into the resulting holes to serve as an easy way to connect the module onto a piece of stripboard.

A first attempt at providing a variable output voltage:
The circuit shown below will allow a PTN78060W to provide a variable output voltage ranging from approx 2.5 Volts up to approx 2 Volts below the supply/input voltage. Only two additional components are needed: a 2N7000 enhancement mode FET (field effect transistor), and an op-amp. Not all op-amps will work in this situation. The problem is that the output of some op-amps cannot drop low enough to provide the full control range. I need to do more tests, but so far have found that NE5534 works well.

Within the range defined above, output voltage (Vo) will track the voltage applied to the non-inverting input of the op-amp (V control).

The circuit works as follows:

As shown in the data sheet, decreasing values for Rset cause the output voltage to increase. An enhancement mode FET such as the 2N7000 behaves rather like a variable resistor ranging in value from near infinite resistance when turned off down to only a few Ohms when fully turned on. Therefore, in the circuit shown below, when the gate of the 2N7000 is at low voltage, the total effective resistance of Rset plus the FET is almost infinate, so output voltage will be at the lowest possible value of approx 2.5 Volts. Conversely, when the 2N7000 is turned fully on, the total resistance of Rset plus the FET is almost equal to that of Rset itself, so that the maximum possible output voltage is determined by Rset alone. The feedback loop to the inverting input of the op-amp then ensures that within the range 2.5 Volts to the maximum voltage determined by Rset, output voltage will track that which is applied to the non-inverting input terminal of the op-amp.

The circuit above could be used to provide a very efficient (ca 95%) variable voltage switch mode regulator circuit that could be used as part of a temperature control loop for a heating system, a Peltier cooling system, or simply to provide a low voltage supply from a 12 volt car battery. Remember that because of the very high operating efficiency of the PTN78060W, no cooling fins are needed!

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