5 Things You Should Know About DC Power

March 7, 2018
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Most modern electronic systems are powered by DC voltages. DC supplies are so common it’s easy to ignore the basics. Here is our list of the five most important things you should know about DC power design.

 

1.      Get the Regulation Right

Voltage regulation is the change in output voltage over the range of loads that can be supplied. Typically, the output voltage of a power system decreases as the output current increases. Although not all systems place a tight limit on the supply voltage, some – for example high performance processors – are unable to operate if the supply deviates even a small amount from the nominal voltage. Voltage regulation is defined as follows:

DC Regulation Equation

Where VNL = output voltage at no load, and VFL = output voltage at full load.

 

Tighter regulation, however, can increase both cost and size of the power system. It’s therefore important to avoid over-specifying the regulation required. Typically, a power component with 1% or lower voltage regulation is referred to as having tight regulation; components with loose regulation might have 2%, 5% or higher regulation.

DCMs offer the ultimate in flexibility, with both loose and tight regulation version available.

 

2.      Small Changes in Efficiency Really Matter

A typical linear regulator delivers a current of a few hundred mA. If the voltage dropped across the regulator is significant, its efficiency can be 50% or less. If we assume a regulator is operating with 75% efficiency, increasing the efficiency by 2% makes little difference, reducing losses by less than 10%.

High-performance regulators and converters are very different. Consider a typical unregulated isolated bus converter, which might operate at around 96% efficiency. Replacing it with a BCM that can operate at up to 97.9% efficiency increases efficiency by 1.9%, but the losses in the converter are almost halved: a huge reduction in the heat that is generated. With high-performance power systems, even small improvements in efficiency make a huge difference to the complexity of the thermal design.

 

3.      Isolation isn’t Just for High Voltage DC or AC-DC Power Systems

Isolation is an important part of many DC power systems. Of course, it’s important to provide isolation for safety in High Voltage DC (HVDC) and AC-DC power supplies, but frequently isolation is needed for other reasons.

Ground loops and noise circulating in systems are challenges that can be addressed by isolation. In a non-isolated system, it’s assumed that the ground plane is at one consistent voltage across the PCB (or the cabling is at the same voltage in the case of a larger system). With the high currents drawn by modern components and the inevitable resistance in any conductor, this is just not true. Isolating a section of the circuit, however, means that it is much easier to reduce the voltage drops to negligible levels.

Another use for isolation is protection, particularly in products designed for harsh electrical environments or that might have long cables connected to them. For example, the power supplies for switches that provide Power over Ethernet (PoE) are usually isolated to prevent damage from spikes on the cable.

Both isolated and non-isolated converters are available,  for example the BCM is effectively an isolated version of the NBM.

 

4.      You Must Think About Safety when Designing with High Voltage DC

High voltage DC is dangerous: touching DC can result in the muscles tightening, preventing the person from letting go of the thing that is electrocuting them. For this reason, SELV (Safety Extra Low Voltage) has been defined.

SELV is less than or equal to 60V, and it should be isolated from the primary power. SELV power systems should also have protection against faults: for example, UL 60950-1 states that a SELV circuit is a “secondary circuit which is so designed and protected that under normal and single fault conditions, its voltages do not exceed a safe value.”

Engineers designing systems that use SELV voltages will find safety approvals much easier to achieve than when using voltages above 60V. Our converters enable designers to step down to SELV from an input of hundreds of volts.

 

5.      DC Isn’t Constant Current

DC (direct current) means that the current only flows one way. But it doesn’t mean that the current is constant. Dealing with the variations in current demanded by the load is one of the big challenges of DC power design. One solution to smooth the demand is adding output capacitance to the power system, but this can only work up to a point. With the very low resistance of an uncharged capacitor, powering on the system, or even recharging the capacitor between pulses, can trigger overcurrent protection and make the power system shut down.

Responding to changes in demand is another challenge. If there is a step change in load, there will be a finite response time from the power system. High frequency switching topologies like the Sine Amplitude Converter (SAC) will respond to the change much faster than other topologies, minimizing the time when insufficient current is being delivered and therefore the output voltage cannot be held up.

 

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