How To Choose The Right Power Supply For Your Vehicle

By Florian Haas

1970
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One of the topics that car owners love to talk about time and again is their vehicle’s performance data. This article provides a guide on how to choose the right power supply for your design, starting with accurate DC/DC converter measurements.

Many people have already said that consumers are being misled when their cars consume more fuel than the manufacturer data specifies. But is misled the right term to use? Or are we dealing with a subject that designers in the electronics industry have long been familiar with, that data collected in the laboratory differs from data collected from actual usage?

How does that happen? Are readings actually incorrect, or are there considerations that need to be taken into account the moment an AC/DC power supply or a DC/DC converter is integrated into a circuit?

Ultimately, it should be clear to everyone that a car tested with no weight inside on a defined test track consumes less than when it is fully-loaded with a family of four on the side of the nearest mountain.

As a rule, personal circumstances, such as budget, will determine which car we buy. But how do we go about choosing the right components to deliver power?

Analysing the requirement

The first thing we should do is define what the respective power supply is required for. This throws up some apparently simple preliminary questions. The key objective is to bring input voltage for the application to a new potential.

  • Should potential be divided or not?
  • What are the input and output voltage ranges, and what is required at which output current?
  • What design do I have space for? For example, do other components on the board dictate this?
  • What is the end product that the circuit is required for?
  • Which regulations need to be followed—industrial, railway or medical?
  • In which environmental conditions will the application be used?
  • How reliable should or must the whole circuit and application be?

So, while a Porsche 911 is a great car, it is easy to understand that it would not be suitable for an expedition through the South American rainforest, where a four-wheel drive jeep would probably be the better option.

But selecting the appropriate power supply is not always as simple as choosing the right car. Once the standard basic questions have been cleared up, we come to the ones that this article intends to investigate more fully:

  • How can I measure correctly and avoid errors?
  • How do I deal with ripple and noise?
  • What happens in the event of inrush current?
  • What do I need to be aware of with respect to electromagnetic compatibility (EMC)?

As an example, Traco Power’s product portfolio includes more than 25 3W DC/DC converters, such as the TVN series with ultra-low ripple and noise, the THM series (certified for medical applications and open-frame variants) or the TMR-WIR series (approved for railway applications with 3000V DC isolation voltage) open up a whole host of possibilities.

We would probably all like a converter that meets all necessary requirements in one go—an off-the-shelf solution, but usually we need to spend more. This is because we need more than one converter to meet the requirements or additional wiring to achieve the values we require.

So, what do we need to establish to find a solution? The first step is to determine which measurement values we need. A simple measurement allows us to easily make a rough assessment—input and output voltage and current on the converter, on the load and possible changes in efficiency.

Avoiding measurement errors

A basic point to remember is that every measurement changes the actual state of the circuit, and any impact needs to be kept as minimal as possible. This means that doing a four-wire measurement even for a simple measurement is advisable. Measuring current and voltage with independent test leads means that the leads’ inherent resistance will have less impact on values.

It is also important to think about end usage. For example, in an operating theatre, there can be 30m wires between the power supply and the actual load. If the load required is 24V, the source needs a higher output voltage to offset voltage loss in the wire. That means that both load and source need to be measured. Fig. 1 shows an example of a classic four-wire voltage measurement at source.

Circuit diagram of a four-wire measurement with source and load
Fig. 1: Circuit diagram of a four-wire measurement with source and load

Establishing and influencing ripple and noise

Why are ripple and noise measured

Regardless of application, ripple and noise of a DC/DC converter could lie in its actual area of operation, for example, in a measuring bridge. As a result, this needs to be considered and evaluated separately.

What are these, and how can these be measured cleanly

We talk about ripple in AC/DC and DC/DC circuits when irregular disturbances are caused by internal circuits. Whereas, noise denotes the peaks that return periodically, produced by the transforming pulsing at switching frequency.

To determine actual values, the probe head must be in direct contact with the pins, the ground ring and the measurement tip, making contact with these. To be able to compare results with a manufacturer’s data, bandwidth on the oscilloscope is limited to 20MHz, a common value for laboratory work (Fig. 2).

Correct application of measurement tips to a DC/DC converter with pins
Fig. 2: Correct application of measurement tips to a DC/DC converter with pins

Usually, noise and ripple can simply be reduced with two parallel-switched capacitors, for example, a 100nF metal-film capacitor and a 10µF electrolytic capacitor. This is done while always bearing in mind that values presented on datasheets can be influenced by other factors during end usage.

Dealing with inrush current

This information is important for ensuring that components upstream are of the correct dimensions.

  • Current depends essentially on switching speed so, ideally, mercury switches should be used in the laboratory.
  • Source should have lowest internal resistance possible.
  • Current is measured with a demagnetised tip.

Ambient temperature also has a big impact on inrush current. For example, use of electrolytic capacitors is highly dependent on temperature.

Inrush voltage of a DC/DC converter in a non-active state (cold-room temperature 25°C)
Fig. 3: Inrush voltage of a DC/DC converter in a non-active state (cold-room temperature 25°C)

Fig. 3 shows an example of inrush voltage in an LED lamp (yellow line). It also shows the voltage pattern in the lamp (purple line). It is good to see the point at which the device is switched on (marked T, in orange), which reaches its maximum value at around 10A and within 10m/s returns to 300mA again.

If you are having issues with inrush voltage affecting a circuit, using a thermistor (NTC) can help.

Electromagnetic compatibility

It is important to establish EMC for the general application. Using a DC/DC converter with an internal filter does not automatically mean that you will adhere to the values specified for the application. This is because EMC compatibility can often be affected by several components.

In many cases, output voltage must be connected with protective earth for safety reasons. This can have a significant impact on EMC. Usually, the power supply manufacturer can offer advice regarding how to adhere to EMC values.

Most power supply manufacturers provide help in the form of suggestions for suitable filters on their websites. For example, at www.tracopower.com these can be downloaded directly from the relevant device’s page. If you cannot find the circuit diagrams for the product you have selected, do not hesitate to contact the manufacturer directly on the phone or by email.

Summary

In conclusion, you do not need to put in a huge effort to establish the power supply components you need for a design if you make use of help from manufacturers and simple resources available.

Before any assessment and selection, it is important to define your requirements clearly. There is a big difference between deciding on what you really need and what you would like.

Coming back to the car manufacturers we referred to at the start of this article, it is important to think about how measurements can be taken correctly, and how every measurement impacts values gathered. Testing strips and laboratory conditions are defined when a circuit is built, but what are the conditions that the circuit will actually be used in?
If there is no suitable device available in the marketplace, maybe, series- or parallel-switching or using a filter might produce the desired results.

Meeting EMC requirements depends heavily on the field of application and its prerequisite conditions, just as with ripple and noise, and inrush current.

It is well-known that there are many manufacturers out there, so the quality as well as price of a specific product can vary greatly.

Is it enough to choose a simple, unbranded, mass-produced device, or will trying to save money come back to bite you further down the line? Do you always need to go for the high-end variant? The final decision is down to you, regardless of any recommendations.


Florian Haas is director of marketing, Traco Power Group

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