Watkins-johnson converter

What is a Watkins-Johnson Converter?

A watkins-johnson converter is a bipolar circuit which steps up voltage with a negative polarity and steps it down with a positive polarity. The Watkins-Johnson converter comprises a tapped inductor, switch, source voltage and an output load. To reduce the ripples caused in the output voltage, a capacitor has been added across the load. The transfer ratio of the Watkins-Johnson converter is as follows:

Vo/Vin= 2-D/D.

watkins-johnson converter

Where D represents the duty cycle, and Vo and Vin stand for output and input voltage respectively.

The duty cycle ranges from 0 to 1. For instance, if we put 0.5 duty cycles, which means the switch ticks 50 percent for the whole time interval. Only the positive polarity can be exploited due to the single switch. If both polarities are required, then an active output load must be used. The Watkins-Johnson Converter can be used as an inverter with different types of topology.

History of watkins-johnson converter

Due to the availability of semiconductor switches in the 1950s, the landscape for electronics circuits changed dramatically. This prompted a whole new field for research and different types of circuits started to pop up. However, the major breakthrough came in the 60s due to requirements by the aerospace industries that required a lightweight and complex system for voltage step up. The boost converter is a SMPS(switched mode power supply) for which R.R middlebrook from caltech in 1977 published the simple model. The Watkin-johnson was initially invented by the research team at the Watkin-Johnson Company. 

Advantages and Disadvantages of watkins-johnson converter

The Watkin-Johnson converter is a highly efficient system with a small number of countable passive and active elements. However, what makes it different is the use of tapped inductors, which provide solid safety to devices not found in the rest of non-isolated converters. 

The input current is continuous which is a plus point for devices such as batteries. No problem of a floating switch exists as it is grounded, if the input to tap topology is considered. The control strategy becomes easier due to the output voltage being of a positive polarity. The input capacitor current is discontinuous which results in a heavy capacitor size and electromagnetic interference issues. However, the practical circuit will be heavy due to the tapped inductor in the mix. If coupled inductors come in the mix for inverter circuits, we may see an even bigger coiled inductor due to the requirements.


This small circuit is a major device in solar power systems. Apart from that, due to its stability, it is used in certain vehicles. Drone technology has become a powerhouse in the recent decade, and to reduce the size of the system, the usage of such circuits have increased. Watkin-Johnson converter is a popular topology for different types of drone models. Apart from that, the aerospace industry was one of the main reasons these converters came into existence. It is used in satellites as a power amplifier due to its small size and high efficiency.


The Watkins-Johnson converter works in two states, when the switch is on and off.  When the switch closes, the battery supplies the power to the inductor, which gets charged and becomes an energy tank. While also moving across the load side path as the diode path is not accessible.

After the charging of the inductor, the switch turns off according to the duty ratio. For example, if the duty ratio is 50 percent, then the switch will be open for half of the total duration. 

In the off state the switch is open, thus the battery moves across the tapped inductor with a negative polarity across the diode. The greater the duty ratio, the more charged the inductor is and the higher will be the reservoir energy in the inductor. 

Prototype creation.

It is really easy to build a Watkins-Johnson converter at home. First get the wires and essential active and passive elements such as tapped inductors( hard to get in some areas, maybe custom build), capacitors and resistors.

After that, a mosfet switch, a diode and a battery would be necessary. Lastly a micro controller such as arduino UNO or Mega. This device is the brain for the whole system. It is where the code is stored, plus, a surface knowledge of coding in C++ would be beneficial for writing the delay code for the switch.

Connect the wires and elements to the micro controller which will be powered by the battery, if it is a large one. The micro controller has a pin which can be either used as inputs or outputs.

Apart from that, a more professional method is to employ a PCB designer to build a compact and low cost converter on a small piece of board. If one wants to learn PCB designing, Altium or Eagle is an easy way to start. I would prefer the former due to its nice graphical interface, however, both can do the job.

However, before designing one must keep in mind that it cannot change the duty ratio without changing the simple delay program. If one aspires for such a conditionality, then the ability to create a complex programming code must be within their grasp.

This is simple stuff, but going hardcore, one can even consider isolating the input from the output by using a transformer or a tapped inductor, however, that is the work of a true professional and making a DIY isolated boost converter is nearly impossible unless the individual is well versed in that field.


The Watkins-Johnson converter is a tapped inductor voltage step up and down bipolar system. It is highly efficient and compact, which is much more flexible compared to linear or shunt voltage regulators. It can withstand small current or voltage fluctuations. It is very easy to build and even a layman with surface level knowledge in C++ programming language can create a untidy prototype in home, however, he must have a neat knowledge of tapped inductors due to the complexity of such an element. In the end, if one knows what the use of this device is, he can modify it for a wide range of applications. For greater in depth analysis, one should check out research articles by Y. Darroman on tapped inductor converters or Fundamental of power electronics by Erickson.

Read more about this