In this article we will take a brief look at SMPS terminology and a few basic design paradigms used in modern PC power supply (Power supply Unit, PSU) designs. In part one of this five part series, we look at the basics of the input features, up to the inrush current limiting stage. The purpose of this article is to demystify industry terminology and construction.
The PC Power supply is one of the most important com- ponents in a well built PC. Consequently, this component has been accorded a rather important status by computer enthusiasts. In this day of power hungry parts such as video cards and multi-core processors, it is all the more important to have a good quality PSU.
Before making a purchase, it would well serve an enthusiast to understand how a typical PSU works and what are the things one should look for in a good quality unit. In this five part series, we intend to look at the features implemented in modern PSU's. In this article, we look at the input side of the PSU.
In Part Two, we will look at Soft-Start, start up Over/Under voltage protection, overcurrent protection. In part three, we will look at output current limiting, output filtering/isolation/noise and snubber networks. In part four, we will look at PFC and practical identification and characteristics of the critical components in use. In part five we will completely take apart and analyze a PSU.
This is going to be a fun project so please hang in there till we get to part five!
Quite often one can find words such as "robust unit", "build quality" and "well populated" being used to describe the modern computer power supply. As computer enthusiasts have come to realize, these terms very much reflect the critical nature of the PC Power supply in the context of powering an overclocked rig. This section will discuss, in brief, the various input requirements specified by international standards and their importance in modern PC Power supply design.
A. Input Transient Voltage Protection and Electromagnetic Compatibility
A transient is a short lived electrical phenomena whose time scale depends on the context.
In case of PSU's, transients can be either voltage or current spikes. IEEE Standard 587-1980 classifies various stress locations and investigates this phenomena in some detail. Frequencies above 30 MHz tend to radiate directly from the generating circuits, while those below 30 MHz are usually conducted by the AC line and other connections.
The IEEE standard recommends a 6 kV peak amplitude for a damped sinusoidal oscillation as shown in the figure. PSU's must filter out these high voltage transients.
The ideal transient suppression device would be an open circuit at normal voltages and would conduct without delay at a certain voltage beyond spec and clamp the voltage at some specified level. It should also be able to handle unlimited surges or spikes and would not break down.
However, no such ideal suppression device exists.
PSU manufacturers try to approach an ideal setting by using a combination of various suppression devices. We will look at a few of them in brief:
Metal-Oxide Varistors (MOV's): MOV's are voltage de- pendent resistors. At voltages below a certain level (called turnover voltage), they have very high resistance and almost act as an open circuit.
When the voltage ac cross it increases beyond this turnover voltage, the resistance rapidly decreases so it can allow the surge current to be taken away from the rest of the circuitry. Their clamping action as required by the IEEE standard is very poor for high current transients. These devices age quickly and such stress related degradation cannot be easily measured or detected by visual inspection. They are usually used in conjunction with other suppression devices.
Transient Protection Diodes (TPD's): These are usually silicon based diodes which have very good voltage clamping action. They are expensive and are seldom used. When these diodes fail, they fail as a short circuit, which usually means the PSU's fuse will blow.
Transient Suppression Capacitors: IEEE-587 identifies two types of transients: Transients can either occur between live and neutral lines or between live/neutral and ground.
Suppression capacitors are designated as Type-X and Type-Y respectively. These transients are also called symmetric and asymmetric transients. While X capacitors can be of any value, Y capacitors are kept small and are usually limited to 4700 pF. Typical X capacitor values are 0.1 to 1.0 µF. In the US, UL standards are most widely employed and capacitors should adhere to these standards (refer to the Appendix for more information).
Transient suppression Inductors: To suppress EMI, a typical filter will include common mode inductors, differential mode inductors and X and Y capacitors. The Y capacitors and the common mode inductors contribute to the attenuation of the common mode noise.
The inductors become high impedances to the high frequency noise and either reflect or absorb the noise, while the capacitors become low impedance paths to ground and redirect the noise away from the main line. To be effective, the common mode inductor must provide the proper impedance over the switching frequency range.
Common mode inductors are wound with two windings of equal numbers of turns. The windings are placed on the core so that the line currents in each winding create fluxes that are equal in magnitude but opposite in phase. These two fluxes cancel each other, leaving the core in an unbiased state.
The differential mode inductor has only one winding requiring the core to support the entire line current without saturating. Herein lies the great difference between common mode and differential mode inductors. To prevent saturation, the differential mode inductor must be made with a core that has a low effective permeability (gapped ferrites or powder cores). The common mode inductor, however, can use a high permeability material and obtain a very high inductance on a relatively small core.
Faraday Screens: Faraday Screens are copper screens used to eliminate parasitic capacitive coupling between components and RF coupling. These are seldom used in PSU's because of cost and because the case provides adequate shielding. Hence, we will not discuss this any further.
B. Input Fuse Selection
The fuse (fusible wire link) is one of the oldest available protection paradigms. In the most elementary case, it consists of a special wire which melts when excess current flows through it, thereby protecting the device it was designed for. There are several parameters one must look out for while replacing a fuse.
Current Rating: The rated current must exceed the maxi- mum DC RMS (root mean square) current demanded by the circuit. The tolerance depends on the requirements.
Voltage Rating: This is not necessarily related to the supply voltage but generally is a measure of its ability to extinguish the arc that is generated when the fuse blows. Failure to select a proper voltage rating will mean a lot of transient energy is let through by arcing and may be a fire hazard.
Slow-Blow Fuse: These are low cost, low-melting point alloys and the fuse element is very thick. They are used in devices where the inrush current is large upon start up, like motors and alternators.
Standard-Blow Fuse: These are usually made of copper in clear glass enclosures and can handle short-term high current transients; selected for short circuit protection in most cases.
Very Fast Acting Fuses: These are used to protect semi- conductor components from spikes and they blow very very quickly. The response time is in the order of micro-seconds.
SCR (Silicon controlled Rectifier) Crowbar Protection: This scheme is as shown below. It is a bit expensive to implement due to the number of components being used. These days, one can find integrated elements which do the job.
This is typified by the use of a large electrolytic reservoir capacitor. Either a single or multiple storage capacitors may be used. Manufacturers like Zippy and Seasonic take the single capacitor approach by picking a low ESR 105 C rated Hitachi for the primary energy bank.
A simple criterion for selecting the input capacitor would be
C = 1.5 µF/W
where C is the effective capacitance of the entire input capacitor bank. There are other important considerations one must look at such as:
RMS Ripple Current Rating: This is actually tied in with the amount of energy lost as heat and depends on the ESR rating of the capacitor. The larger the ESR (Effective Series Resistance), the more energy is dissipated as heat.
Capacitors with larger ESR are prone to heat related failure, as in most cases ESR dramatically increases with rising ambient temperature. If you every wondered why your PSU blew its caps out, this is the reason.
Ripple Voltage: This requirement defines the minimum capacitor value during short timescales. The output from the rectifier assembly needs to be be smoothed out and this is accomplished by a capacitor.
Undersized caps are frequently used when economy overrides design prudence and this man- ifests as undesirable voltage ripple. Quite often, the following stage is a voltage regulator based on DC-DC conversion. This is dependent on having a clean DC input in the first place. If the input has excessive ripple, the PWM assembly will be under unnecessary stress.
Holdup Time: It is the minimum time for which the supply will maintain the output voltage within design limits when the input supply is removed or falls below the input regulation limits. Holdup time is a very important parameter and plays a big role in the choice of the input capacitor and is given as:
where Ei is the energy used during holdup time and the voltages are supply and failure voltages respectively.
In AC/DC power converters above a few watts, a large inrush current flows when the input capacitors are suddenly charged during the initial application of power. If unrestricted, this current can easily exceed 50 A at the peak of the AC cycle.
This high inrush current severely stresses the converter's fuse, input rectifiers and power switch. It can significantly reduce the reliability and life expectancy of the modules. Most PSU's use some form of bridge rectification to convert the AC from the mains to DC (with input capacitors).
This is usually in the form of a diode arrangement or an integrated bridge rectifier assembly. If the line input is switched to this type of rectifier-capacitor assembly, very large current is drawn by all components in a short timescale. Hence, there is a need for "inrush control" and is usually some form of a series limiting resistive device.
A few techniques used for this purpose are:
Using Resistors: For very small power supplies, a few watts at most, adding a resistor in series with the line is a simple and practical solution to limit the inrush current. The large resistance required to limit peak inrush current causes too great a loss in efficiency to be used in higher wattage power supplies. It is sufficient to use resistors immediately after the surge suppression filter section and between the bridge rectifier/capacitor stages.
Using NTC Resistors/Thermistors in series with line: Many power supply manufacturers use a negative temperature coefficient (NTC) resistor in series with the line. An NTC resistor offers tens of ohms of resistance when cool, dropping to less than one ohm as its temperature increases.
If the power supply is cool when turned on, the NTC provides good inrush current limiting. Its effect on efficiency is reduced as the power supply warms up. However, this approach is not effective over large temperature extremes. A power supply used outdoors in the northern winter may never warm up enough for the NTC resistance to drop. Conversely, a supply in the hot summer sun will be very warm even with the power off, so that the warm NTC resistor will fail to provide adequate inrush current protection on start up.
An NTC resistor can also be problematic when a user turns the system off and then immediately switches it back on again. The capacitor voltage may drop, but the NTC resistor will not cool quickly enough to provide inrush current protection. That is why it is bad to switch on and off the PSU rapidly.
Using an Active Limiting device: For a high powered PC power-supply (anything available today, really), it is more energy efficient to have the limiting device shorted out of the circuit, unless needed. This will reduce operational losses when the unit is in steady state.
A TRIAC is usually the active device of choice in such an implementation. The SCR or TRIAC approach limits the inrush current by progressively varying the phase of the AC line voltage at which the SCR/TRIAC is switched on during start up. The instantaneous line voltage at which the SCR is activated is incrementally higher at each subsequent cycle ensuring that the difference between the line voltage and the output bulk capacitor voltage is small enough to result in negligible inrush current.
This kind of circuit is considered to be non-power dissipating. However, the triggering circuit itself consumes power and the SCR has to be fed with a continuous pulse to prevent it switching off due to line glitches The power needed for this circuit is therefore not insignificant.
These methods are "one shot" which means that (unless the input power is cycled on/off or additional, complex circuitry is added) they cannot limit inrush due to subsequent disturbances on the power supply line after initial power on.
We have seen in brief, a few important characteristics and requirements of modern PSU's. We will be looking at more characteristics in the next part of this article.
To be continued...
Auf Wiedersehen!
IV. REFERENCES
1) SMPS Handbook by Keith Billings
2) Tech notes, Bear Power supplies
3) Meeting Military Requirements for EMI & Transient Voltage Spike Suppression, xp-military.com
4) Common Mode Inductors for EMI Filters Require Care- ful Attention to Core Material Selection by Robert West, Magnetics, Division of Spang & Co., Butler, Pennsyl- vania
5) Capacitors for RFI Supression of the AC line: Basic Facts,300 Tri-State International, Su. 375 Lincolnshire, IL 60069 847 948 9511 Copyright c 1996 Evox-Rifa, Inc
6) Ripple Current Capabilities 2004 KEMET Technical Update by John Prymak Applications Manager, P.O. Box 5928 Greenville, SC 29606 Phone (864) 963-6300 Fax (864) 963-66521 WNW.kemet.com.
7) A new approach to PFC Inrush protection by Linus Liu, Copyright c 2006 Astec International Limited
APPENDIX
Underwriters Laboratory (Trivia)
The United States actually does not have a national safety agency. Underwriters Laboratories (UL) is a private corpo- ration. Various federal and state laws require that electronic products be listed with any Nationally Recognized Testing Lab (NRTL) of which UL is the most widely known. NRTLs typically test end products to UL standards (plus those of other countries) and very few test primary components such as capacitors.
UL was established over 100 years ago by a group of insurance companies to promote product safety as a means of reducing insurance claims. Accordingly UL's focus has generally been on traditional consumer products - this is the case even today.
An RFI capacitor in an electronic ballast has less stringent requirements. The same was true of RFI capacitors in switched mode power supplies until UL adopted the IEC950 standard. At the present time there are two UL standards related to RFI capacitors: UL1414 and UL1283. UL1283 is actually a standard for potted RFI filters.
The only meaningful reason to have UL1283 recognition for a capacitor is to demonstrate the capacitor's ability to survive the tests that the filter must undergo. UL1283 recognition of a capacitor is not required by any UL equipment standard. The requirements are not very stringent, consisting primarily of a dielectric withstand test.
UL1414 is specifically required for television and radio receivers and certain telecommunications equipment. The tests are quite stringent. A capacitor may be rated for either 125 or 250 VAC (nominal) at 85ºC. The requirements of UL1414 at 250 VAC are summarized:
- A 1500 VAC dielectric withstand test for 1 minute.
- Be subjected to 50 discharges from a cap charged to 10 kV through a 1000 W resistor, then pass a 1 kV dielectric withstand test.
- A 1008 H endurance test at +85ºC with an applied voltage of 440 VAC 60 Hz. Once per hour, for 0.1 s, the voltage is raised to 880 VAC.
- A passive flammability test.
- An active flammability/expulsion test.
For the most part, ferrites are the material of choice for common mode inductors and they are divided into two groups viz., Nickel-Zinc and Manganese-Zinc.
Nickel Zinc materials are characterized by low initial permeability's (<> 100 MHz). Manganese Zinc materials, on the other hand, can attain permeability's in excess of 15,000 µ but may start to "roll-off" at frequencies as low as 20 kHz.
Because of their low initial permeability's, nickel Zinc materials will not produce a high impedance at low frequencies. They are most often used when the majority of unwanted noise is greater than 10 or 20 MHz. Manganese Zinc materials, however, offer very high permeabilties at low frequencies and are very well suited to EMI suppression in the 10 kHz through 50 MHz range.
High permeability ferrites come in many different shapes like Toroids, E cores, Pot Cores, RMs, EPs, etc; but for the most part, common mode filters are wound on toroid's. There are two main reasons for using toroids:
First, toroids are generally less expensive than the other shapes because they are one piece, whereas other shapes require two halves. When cores come in two halves, they must be flat ground on their mating surfaces to make them smooth and to minimize the air gap between them. Furthermore, high permeability cores often require an additional lapping procedure to make them even smoother (this produces a mirror-like finish). Toroids require none of these extra manufacturing steps.
Second, toroids have the highest effective permeability of any core shape. The two-piece construction of the other shapes introduces an air gap between the halves, which lowers the effective permeability of the set (typically by about 30%). Lapping improves this but does not eliminate it. Because toroids are made as one piece, they do not have an air gap and do not suffer a reduction in effective permeability.
Toroids do have one disadvantage, their high winding cost.
Bobbins, which are available for the other shapes, can be wound quickly and economically. Toroids require special wind- ing machines or must be wound by hand, making the per-piece winding cost higher. Fortunately, the number of turns on com- mon mode inductors is usually quite low, so the winding costs do not become too prohibitive.
For these reasons, toroids are the geometry of choice in common mode inductors. There are several other material characteristics and design considerations one must take into account while selecting inductors, but that would be beyond the scope of this article.
1 comment:
Great thoughts you got there, believe I may possibly try just some of it throughout my daily life.
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