Conversely, a transformer that isolates the output of a switching regulator can be very small because it is operating at the switching regulator frequency of typically 50 kHz or more. The use of multilayer insulated wire (also known as triple-insulated wire or Rubadue wire) is recommended for the transformer windings. When using triple-insulated wire, it is important to remember that such wire must satisfy the requirements described in Annex U of EN 60950. A simple example of an isolated LED driver circuit is given in Fig. 15.1.

If accurate current control is needed, additional electronics to control the LED current is needed and some form of isolated feedback is required. An optocoupler, which is an LED and a phototransistor in a single package, is usually used to provide the feedback signal. The LED in the optocoupler is powered from the transformer secondary winding and is controlled using a precision voltage reference and a current sense circuit, to accurately control the output current. The phototransistor enables and disables the switching of power to the primary winding, as required.

For products connected to AC mains supplies, 1500 V RMS (50 or 60 Hz) isolation is usually required. Products for medical applications usually require a higher isolation voltage with strict limits on leakage current; LED lamps are sometimes found in hospital operating theaters and in other medical applications.

Fuses have a specified voltage rating. This is the maximum circuit voltage at which a fuse can be relied upon to safely interrupt an overcurrent. At voltages higher than the rating, a fuse may not be able to suppress the internal arcing that occurs after the fuse link melts. In electronic circuits, where limiting impedances ensure that the fault current is kept low so that a destructive arc cannot occur, fuses may sometimes be used beyond their specified voltage rating.

Fast-acting fuses are designed to open very quickly during an overload. They are not designed to withstand the inrush current or switch-on surge that occurs in some equipment. If an inrush limiter (NTC thermistor) is used, a fast-acting fuse gives the greatest protection. The overload current needed to cause a fast-acting fuse to open quickly (about 0.1 s) must be at least double the rated current. An overload of only 35% (e.g., 8 A through a 6-A fuse) could take an hour to open the fuse.

Time delay or slow blow fuses are designed to tolerate the temporary current surges that occur during switch-on, but they still blow quickly if subjected to severe overloads during faults. Slow blow fuses are commonly used in the primary circuits of electronic equipment, where the initial surge can be many times the normal load current. However, if a continuous overload occurs of double the rated current, the fuse will blow within 1 min.

Electronic circuit breakers are also available. These usually latch in the off state when a fault is detected, so that cycling the power supply off and then on again is generally required in order to reset the circuit breaker.

Tyco and others produce a type of fuse that becomes of high impedance when an overcurrent is detected, due to the current’s heating effect, but then the electrical connection is restored once the fuse has cooled down.

The requirements for creepage distance depend upon the application. Some integrated circuits have “no connect” (NC) pins between high and low voltage pins, so that a small piece of solder cannot bridge any two points. I have seen customers cut a slot in their circuit board to allow them to reduce the overall circuit board size. The creepage distance in air is much shorter than the creepage distance on a PCB surface. One way to avoid cutting slots in the PCB is to apply a conformal coating over the assembled PCB. The conformal coating is usually a silicone-based elastomer or a polyurethane varnish. Conformal coating materials must be UL recognized, particularly if a product is to be sold in North America.

When designing an isolated switching LED driver, a typical rule of thumb is to allow 8-mm creepage distance between primary and secondary circuits. A 4-mm creepage distance should be allowed between primary and ground. If these dimensions are allowed for during the design stage, there is a high probability that the product will gain approval with respect to creepage and clearance when the final product is submitted for test.

The working voltages within the circuit must be taken into account, not just the input and output voltages. Transistors and integrated circuits with built-in reinforced insulation (body thicker than 0.4 mm) must also still meet the spacing requirements at their connection pins.

When measuring working voltages, it is important to measure both peak and root mean square (RMS) voltages. The peak value is used to determine the clearance, and the RMS value is used to calculate creepage. Measurements should be accurate and repeatable and should also consider the end application.

Capacitors connected from the AC line to earth must be “Y-rated,” usually Y2. The typical DC operating voltage of Y2 capacitors is 1500 V. These capacitors normally have low capacitance (say 2.2 nF) and are usually made with a ceramic or polypropylene dielectric.

After the bridge rectifier, there is no requirement for a special voltage rating of the components because it is deemed that the bridge rectifier will become open circuit under fault conditions. In fact, the bond wires to the silicon die will act like a fuse.

Standard capacitors, rated at 400 or 450 V can be used on the DC side of the bridge rectifier. Since these are not rated for operation above their nominal working voltage, they are often smaller and of lower cost compared to X2 capacitors. For this reason some engineers will place the EMI filter after the bridge rectifier. However, an EMI filter before the bridge rectifier is preferred because it will tend to slow down voltage transients and prevent harmful voltage from reaching more sensitive components. It will also reduce the small transients generated by diodes in the bridge rectifier, caused by sudden changes in current flow during each half cycle of the AC supply, as capacitors on the DC side of the bridge rectifier are charged. A small X2-rated capacitor across the AC side of the bridge rectifier and the remainder of the EMI filter on the DC side of the bridge rectifier is a reasonable compromise.

The European Low Voltage Directive (LVD) is a safety regulation in Europe that covers all products operating from voltages of 50 to 100 V AC and 75 to 1500 V DC. There is a further “catch-all” General Product Safety Directive. These directives require a CE mark to be placed on all goods offered for sale. But to get permission to use the CE mark they must comply with safety standards like EN60950. Note that submodules do not require CE marking, but the overall equipment does. Clearly submodules ought to be safe to operate and any EMI should be low enough so that the final equipment can easily pass approval testing, otherwise the equipment assembler may decide to look elsewhere for his submodules!

To ease the burden in safety testing, many people ensure that their products operate at low voltage. The safety extra low voltage (SELV) requirements are that no touchable conducting parts have a voltage (relative to ground, or across any two points) above 60 V DC, or 42.4 V peak/30 V RMS AC. For example, a DC-powered (boost–buck) Ćuk converter, with 24 V DC input must not have an output above 36 V. This is because the Ćuk produces an inverted output, so the difference between the input and the output is the two voltages added together.

An AC mains–powered LED lamp must be transformer isolated to meet these regulations. The transformer must have primary and secondary windings with good galvanic isolation. As the primary circuit is an internal circuit connected directly to the external AC mains supply, this circuit contains hazardous voltages. The secondary circuit, which has no direct connection to primary power, may or may not be hazardous, depending on the secondary voltage.

Nonhazardous circuits are classified as SELV. The output voltage must be limited to below 60 V, which allows the electrical connections to be “touchable.” If the equipment has an isolated cover, this is not enough to ignore the voltage in the primary circuit since the user could remove the cover. In the event of the cover being removed, there should be a microswitch to disable the equipment; alternatively, a second cover should be placed over the primary circuit. A double-fault (cover broken or removed AND microswitch broken or disabled) has to occur before the user can touch a potentially lethal voltage.

Each part of a circuit must be studied to determine the necessary insulation grade. Table 2G in EN 60950 describes common applications of insulation. By both measuring the working voltage and establishing the pollution degree, the appropriate row and column in Table 2H (and possibly also Table 2J) determine the minimum clearance distance required. For one test, the internal components and parts in both primary and secondary circuits are subjected to a steady force of 10 N, which may bend the PCB, but minimum clearance distances must still be maintained during the test. For example, to achieve the creepage and clearance demands between a primary circuit and a nongrounded SELV circuit usually requires reinforced insulation.

DC input products, however, can be treated in one of two ways. They can be considered as being fed by an extra-low-voltage circuit, or as hazardous secondary voltages. This would mean that the clearances could be calculated using Table III in EN 60950 rather than Table I, thus requiring slightly smaller clearance distances. DC input products may also be considered as being fed by SELV secondary circuits, depending upon the end application. If isolation is needed, then Table III of the standard is used. However, if isolation is not required in the end application, clearances can be ignored and only operational insulation is required.

As electronic circuits continue to get smaller, it is more important than ever to have a good PCB design that not only reduces electromagnetic interference emissions, but also reduces creepage and clearance problems. Where shortage of space is an issue, especially between primary and SELV circuits, techniques, such as slots cut through the PCB can be used to achieve the desired creepage distance. Slots must be wider than 1 mm to be considered acceptable. A lack of adequate creepage and clearance distance between a component in a primary circuit and a component in the SELV circuit is a common cause of product failure.