The choice of capacitors for pulse voltage converters. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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The choice of capacitors for pulse voltage converters. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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When constructing a pulsed PN, the question immediately arises - which capacitors to put both on the output of the rectifier and on the support of the primary winding of the transformer. The question is not idle - it was not easy to find good pulse capacitors.

First, let's decide what capacitors we need. By voltage on the secondary bus - 35, 50 or 63V, flexible outputs, by overall restrictions - with a capacity of 1000 microfarads (50-63V), 2200 microfarads (35V). To simplify, I will limit myself to a single rating of 1000uF at 50V, which approximately corresponds to the size of 16 * 36mm for standard electrolytes. Let's take the GS (Standard, 105С) series of the Taiwanese company Ark Electronic, widely available in Moscow, as an absolute starting point. For comparison, let's put the impulse Ark SZ and Mallory purebred Americans side by side.

Family album of electrolytes. Key parameters

Constructive. We immediately cut off large cans for screw terminals, subminiature (4-8mm) containers, containers with axial leads (the current loop lengthens, inefficient installation) - limited to either standard flexible radial leads or hard hooks under the terminal, they can also be soldered to the board.

Temperature range, service life (Lifetime), reliability category (Reliability Grade)

The service life and time between failures are closely related to the upper limit of the temperature range. Depending on the composition and technology of "packaging" of the electrolyte, the upper limit is set at +85C (standard), +105C (increased), +125-140C (high-temperature containers). The service life is set precisely for this temperature, it is 1000 hours for standard types and is defined as the time during which the parameters of the capacitor, charged to the maximum permissible constant operating voltage, are guaranteed to remain in the "green zone". As a rule, deviations are controlled: capacitance (20%), loss tangent (not higher than +50%) and leakage current (not higher than the guaranteed maximum). For pulsed electrolytes, an increase in ESR and (or) total impedance is also normalized.

Selection of capacitors for switching voltage converters

(click to enlarge)

1000 hours is a ridiculously short period, albeit a deliberately underestimated one. But with a decrease in temperature for every 10 degrees up to + 25C, the service life doubles. Thus, a container marked 105C under equal conditions is 4 times more durable than a container marked 85C! Considering the stressful living conditions in a car amplifier, we will limit ourselves to normalized capacities +105C and above. Capacitors of increased reliability/durability (on-board) are also standardized for periods of more than 1000 hours, up to 20.000 hours, but this is a shortage. Due to technological reasons, it is difficult to obtain high reliability in a miniature case, so many advanced series guarantee 5000+ hours for 10mm diameter and above, and 8mm and below - only 2000 hours.

Leakage Current the capacitor is not important for us. There are capacities specially rated for low leakage current. The order of currents (for the selected rating at the limit U and T) is -

Standard (Ark GS 105C) : I(mA) < 0.03 C(mF)U(V) = 1.5 mA

For switching power supply (Ark SZ 105C): I(mA) < 0.01 C(mF)U(V) + 0.003 = 0.5 mA

Improved leakage current (Ark SL 105C) : I(mA) < 0.002 C(mF)U(V) = 0.1 mA

Mallory's leaks are about the same

For a network amplifier with capacitances of the order of 40.000 uF, the leakage current of standard capacitances will be 80mA, the power loss at 63V is 5 watts, which is not so significant, especially since in real life not the maximum voltage is applied to the capacitance, but much less. In a car amplifier, the total capacitance is several times less, so we neglect the uyechka current.

Attention! In bourgeois literature, all dynamic parameters BY DEFAULT are normalized to 120 Hz, not 50 Hz as in GOST.

Loss Tangent (Dissipation Factor) all standard capacitors fit into the range of 0.15-0.25. The "pulse" loss angle tangent is half as much, about 0.06-0.15, with 0.15 corresponding to low operating voltages, and 0.06-0.10 to voltages of 50-100V. It is for this reason that in the input, 12V circuit, immediately before the primary of the transformer, you can see capacities marked + 35.. + 50V, although even taking into account impulse surges, the voltage is sufficient and + 20-25V. At high (above 100-150V) voltages, the loss tangent increases again.

Ripple current limit (Ripple Current) - important for food filters, the more the better! It is determined by the design (ohmic resistance of the plates and leads) and the characteristics of the electrolyte. With an increase in the ripple frequency from about 10 Hz to 1 kHz, the allowable ripple current increases from about 75% to 125-150% of the norm, then for standard capacities, the high self-impedance forcibly limits the current below the norm. With a decrease in temperature to 40-60C, the current rate also increases, but not more than twice.

The order of normalized currents for our capacitor (feel the difference)

Standard (Ark GS 105C) : I(max) = 0.95 A (120Hz 105C)

Standard (Mallory SK 85C) : I(max) = 1.35A (120Hz 85C)

same at 1 kHz, 65C: I(max) = 2.0 A

For switching power supplies (Ark SZ 105C): I(max) = 1.4 A

For switching power supplies (Mallory SXR 105C): I(max) = 0.83 A (120Hz 105C)

same at 120Hz, 65C: I(max) = 1.76 A

same at 100kHz, 105C: I(max) = 1.82 A

same at 100kHz, 65C: I(max) = 3.8 A

In domestic practice, they use the norm of limiting sinus VOLTAGE ripples of 50 Hz on the capacitance. This parameter and ripple current are interchangeable. The voltage is convenient in that for the entire series this parameter alone is sufficient, which depends little on the capacitance. And the current (for a specific rating) is closer to the physical meaning of the processes that destroy the capacitance.

Equivalent series resistance - the main indicator of the suitability of the capacitance for pulsed applications. It is normalized as a rule only for impulse electrolytes.

Standard (Ark GS 105C) : Not standardized

Standard (Mallory SK 85C) : 130mΩ (120Hz 25C)

For switching PSU (Ark SZ 105C): 50 mOhm (100 kHz 20C)

For switching PSU (Mallory SXR 105C): 130 mΩ (100 kHz 25C)

Soviet K50-33 1000uF-63V: 100mΩ at 10-1000kHz - not bad at all! Below 10 kHz, it increases linearly to about 0.75 ohms at 20 Hz. True, the size is 26 * 60 mm, twice as large as the bourgeois ones.

It is believed that by replacing one large electrolyte with many small ones in parallel, one can significantly reduce the impedance. Is it so? Let's compare our 1000uF capacitor with two 470uF capacitors and ten 100uF capacitors. For Ark SZ:

Z (1000) = 50mΩ

Z (470) = 80 mΩ; Z (2*470) = 40 mΩ

Z (100) = 250 mΩ; Z (10*100) = 25 mΩ

First, the misconception that a small capacitance has less resistance than a large capacitance dissipates. No, it's a big one - less. Secondly, there is an effect, but it manifests itself only with a large denomination gap, and incorrect routing of traces can even worsen the situation. Let's check on Mallory SXR:

Z (1000) = 130mΩ

Z (470) = 280 mΩ; Z (2*470) = 140 mΩ

Z (100) = 1330 mΩ; Z (10*100) = 133 mΩ

Oops! No effect. Moreover, the absolute value of resistance is several times worse than the Taiwanese. Either someone is lying, or someone is playing it safe. And what if we check on large banks - for example, we collect 0.2 F from the Mallory CGR series capacitors for 20V

51mF: Z(51mF) = 8.5mΩ, Z(4*51mF) = 2.2mΩ, total current limit 4*22=88A

20mF: Z(20mF) = 8.5mΩ, Z(10*20mF) = 0.85mΩ, total current limit 10*17=170A

7.7mF: Z(7.7mF) = 23mΩ, Z(26*7.7mF) = 0.88mΩ, total current limit 26*8=200A

The effect appears only at the highest ratings of the series (from 51 to 20 mF), where the total impedance of the bank is determined by the resistance of the leads, and disappears at "small" ratings, when the impedance begins to increase inversely with the capacitance. And the inductance of the installation, most likely, will lead to a deterioration in the parameters, we are talking about milliOhms and nanoHenry. So, when working with a specific series, if you please, either look for detailed documentation or measure the capacitance - but how to do it for currents of hundreds of amperes in kitchen conditions .... only Tyk's time-tested method remains.

Special types of electrolytes - Bourgeois terminology

Audio grade- vague term. It includes both high-linear capacitances for the power filter with a high discharge current, and all kinds of non-polar "for crossovers", "feed-through", etc. bastards of mass technology. I have just included in the table what fits the first category

Ballast - ballast for LDS and motors, 160-400V, up to 22 uF. Impulse indicators - average.

Computer Grade - has nothing to do with impulse parameters! This is an intermediate reliability standard, better than domestic but worse than on-board, as a rule, 2000-3000 hours of operation are standardized with somewhat tighter tolerances for parameter care.

Deflection - for horizontal deflection system, 25..100V, capacitance up to 100 uF. Impulse indicators are good.

High energy - high energy (large current) of a single discharge, in contrast to High Ripple Current - high ripple current

High Temperature - ultra-high reliability (on-board), specified for 125C and above. The volume and weight are 4-8 times larger than the standard.

photoflash - for flashlights, 300V, 1-100uF, low leakage current, standard impulse readings.

Notes on Soviet Capacitors

Many of them are rated to fail 5000-10000 hours at 85C. However, the "failure" specifications include a 50% drop in capacitance, a threefold increase in the loss tangent and leakage, which is not comparable with modern bourgeois standards.

Already mentioned К50-33 is produced (still - Severo-Zadonsky plant) is produced with 4 axial leads, which, with a capacitor length of 60-90 mm, inflates the current loop (in the primary circuit) to an unacceptable length. The impedance is normalized to 10-1000 kHz and ranges from 30 to 100 mΩ for all ratings - this is good. Worse, during the operation we will allow its threefold growth. The minimum time between failures (taking into account the above limits) is 2000 hours at 85C, 5000 hours at 70C. This is the only truly high-frequency electrolyte in the Soviet nomenclature. The so-called "pulse" aluminum tanks K50I-1, K50-3I, 13, 17, 21, 23 and their relatives K50-19 starting capacitors are designed for circuits from 150 to 1000V and are not applicable to our tasks. Their resistance is not standardized.

Tantalum "pills" K53-28 are produced up to the maximum rating of 10mkF * 40V, 68mkF * 16V also with axial leads. At the same time, the total resistance is 0.4-10 Ohm (0.4 Ohm is just for 10uF * 40V, with tablet dimensions of 15 * 12 * 5mm). Niobium K53-27, also with axial leads, are produced with maximum ratings of 10uF*40V, 47uF*20V, 220uF*16V. The resistance is normalized at a frequency of 200 kHz (for these values 0.3-1.0 Ohm). As for the widely used semiconductor Al, Nb, Tl capacitors K53 of other series - none of them is normalized for resistance (or current) at high frequencies, so there is nothing to talk about. And the specific capacity is unacceptably low.

So what to put?

Here is a selection of types of aluminum capacitors from companies represented at Moscow bazaars (excluding bipolar and screw-mounted ones). No uniformity! "Banks" are marked with an asterisk, all others - with flexible conclusions. Well, where to look - you yourself will figure it out, search and find.

impulse indicators	Impulse			Standard
Reliability / Audio Grade	High Rel (5000+ hours)			High Rel, Computer grade	Audio Grade	other
Temperature class	Long Life 105C		85S	Long Life 105C
Manufacturer
the USSR			K50-33 (1MHz)
Ark Electronic		SZ		GA GR		SA-SS LGS LGB
CapXon	SZ GL (over 8mm)	GL (5-8mm)		TH KM		SK-SS GS LL LP HP
Elna	RSG RJB RJH RJJ	RSE RJ3		RSL RKA LPK LPH LPG LPT LPX	ROA(Cerafine) ROS (Silmic) ROD R2O R2A RA2 RA3 *LPO	LP4 LP5
Jamicon	TL WG WL			TM WB TH HS HP *RP	LA*AP	SH-SM LP LS *KP
Mallory		SXR VPR		SEK SH *LP		SK SS LPW LPX
Nichicon				DQ GJ GN GR GY GZ	*KG	LN LS LU GU
Samsung	TMQ TMF TMZ	STL trF trQ		UHT TMB*HMB	*PST(?)	SSE-SSL LN LN7 USL SMM SEM ST-STM trB PS SMS SMU HRB-HRL
Samwha	RZ	RX WD NH	NF GF GT	RS RW *HB	AD-AU(?)	RC-RR RV NP NS and all on BSTQ HC HE *CU

Publication: klausmobile.narod.ru

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