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current_doubler [2015/04/15 11:46]
stan_zurek [See also]
current_doubler [2020/07/19 00:04] (current)
stan_zurek [Definition]
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 +====== Current doubler ======
 +
 +|< 100% >|
 +| //[[user/Stan Zurek]], Current doubler, [[http://Encyclopedia-Magnetica.com|Encyclopedia-Magnetica.com]], {accessed on @YEAR@-@MONTH@-@DAY@}// |
 +
 +===== Definition =====
 +
 +**Current doubler**, **current-doubler rectifier** or **hybridge**[([[http://google.com/patents/US6400580|Bowman et al., System and method for reducing a DC magnetic flux bias in a transformer and power converter employing the same, Patent US6400580, 2002]])] - a [[topology]] used in [[switched-mode power supply|switched-mode power supplies]] (SMPS) for [[full-wave rectification]] of an alternating current.[(Ioinovici>[[http://goole.com/books?isbn=9781118443361|Adrian Ioinovici, Power Electronics and Energy Conversion Systems, Fundamentals and Hard-switching Converters, John Wiley & Sons, 2013, ISBN 9781118443361, p. 706]])][(Balogh>[[http://www.ti.com/lit/an/slua121/slua121.pdf|Laszlo Balogh, The Current-Doubler Rectifier: An Alternative Rectification Technique For Push-Pull And Bridge Converters, Design Note DN-63, Unitrode, 1999, {accessed 2014-10-06}]])]
 +
 +<box 60% left #f0f0f0>
 +Current doubler designed to produce 200 A DC implemented with [[planar inductor|planar inductors]]
 +[[file/current_doubler_magnetica.jpg|{{current_doubler_magnetica.jpg}}]]
 +{{page>insert/by_SZ}}
 +</box>
 +
 +The current doubler comprises two output inductors not [[magnetic coupling|magnetically coupled]] to each other, but connected in a circuit similar to a [[centre-tap rectifier]]. The name "current doubler" refers to the output current, whose value is twice the currents in each inductor (when operating in a [[continuous mode]]).
 +
 +The current doubler topology was first used with [[vacuum tube|vacuum tubes]] in 1930's, and also described in 1987 a Danish patent DK1987000003826 issued to O.S. Seiersen.[([[http://www.smps.com/knowledge/Idoubler/idoubler.shtml|Constantin Darius Livescu, Current Doubler Topology - Myth and Reality, http://www.smps.com, {accessed 24 Sept 2013}]])]
 +
 +<WRAP clear></WRAP>
 +{{page>insert/paypal}}
 +
 +===== Advantage over ordinary centre-tap rectifiers =====
 +In a low-voltage high-current [[SMPS]], the [[secondary voltage]] is often rectified with the help of [[centre-tapped configuration]]. The main advantage is that at any instant of time just one diode is rectifying (as compared to the full-wave bridge rectifier). This improves energy efficiency, because the forward voltage drop on the diode (e.g. 0.7 V) can be a significant proportion of the output voltage (e.g. 3 V). The main disadvantage of such approach is that the [[transformer]] is oversized, because only half of the [[secondary winding]] conducts at any time.[([[http://google.com/books?isbn=9780074519653|N. N. Bhargava, N. N. Bhargava S. C. Gupta D. C. Kulshreshtha, Basic Electronics and Linear Circuits, Tata McGraw-Hill Education, 1984, ISBN 9780074519653, p. 89]])]
 +
 +For low voltage applications the secondary winding might have very few turns (usually just one or two), which complicates selection of an appropriate [[turn ratio]] of the transformer. 
 +
 +Current doubler alleviates such problems. Finer control over turns ratio is possible, because the secondary voltage of the transformer is higher and the current is doubled outside of the transformer.[(Balogh)]
 +
 +Some manufacturers supply [[magnetic component|magnetic components]] or accessories for making dedicated current doublers.[([[http://www.paytongroup.com/webfiles/files/4%20Planar%20Transformers%20Size%20250.pdf|Transformer type T250 AC P.N. 101044, Transformer and inductors, Payton Planar {accessed 3 Oct 2013}]])]
 +
 +===== Circuit diagram =====
 +<box 30% right #f0f0f0>
 +Typical circuit diagram of a current doubler
 +[[file/current_doubler_diagram_magnetica.png|{{current_doubler_diagram_magnetica.png}}]]
 +{{page>insert/by_SZ}}
 +</box>
 +
 +A transformer //**Tx**// is driven in a bipolar way ([[push-pull]], [[half-bridge]] or [[full-bridge]]). Two identical [[inductor|inductors]] //**L1**// and //**L2**// are connected across the secondary winding of //**Tx**//. These inductors are not [[magnetic coupling|magnetically coupled]] to each other or to the main transformer.
 +
 +Because of bipolarity, an alternating voltage is driven across the two inductors connected in series. The load //**R**// is connected between the central point of the inductors (similar to a regular centre-tap configuration) and the shorted output of the two diodes //**D1**// and //**D2**//. The low-pass filter can be implemented as the capacitor //**C**//, or as an additional output inductor (not shown).
 +
 +===== Operation =====
 +
 +The name "current doubler" comes from the fact that the average output current of the whole device is twice the average currents in each of the output inductors. 
 +
 +<box 30% left #f0f0f0>
 +Current and voltage waveforms - the output current is a sum of the two inductor currents
 +[[file/current_doubler_waveforms_magnetica.png|{{current_doubler_waveforms_magnetica.png}}]]
 +{{page>insert/by_SZ}}
 +</box>
 +
 +However, the [[RMS value]] which are responsible for temperature increase of the [[winding]]s depend on the [[DC offset]] in each current. If the [[current ripple]] is very high, then the RMS values increase accordingly and become comparable with the RMS value of the total output current. Under such conditions the "doubling" property of the topology is not utilised properly.
 +
 +For this reason, the current doubler is usually operated in [[continuous conduction mode]], so that the currents in the two output inductors keep flowing at all times, with large DC offsets and relatively small ripple, which reduces the RMS values. This is achieved by using proportionally larger [[inductance]] of the output inductors. Under such conditions the average value of the output current remains the same, but its RMS is twice the inductor currents. Additionally, the ripple in the output current is also reduced by half. This occurs because most of the time one current increases and the other decreases, so the ripple in their sum appears accordingly reduced, as shown in the waveforms.
 +
 +<WRAP clear></WRAP>
 +<box 40% left #f0f0f0>
 +Typical current waveforms with low DC offset ([[inductance]] too low) 
 +[[file/current_doubler_low_dc_offset_magnetica.png|{{current_doubler_low_dc_offset_magnetica.png}}]]
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 +</box>
 +<box 40% left #f0f0f0>
 +Current waveforms with high DC offset (increased inductance)
 +[[file/current_doubler_high_dc_offset_magnetica.png|{{current_doubler_high_dc_offset_magnetica.png}}]]
 +{{page>insert/by_SZ}}
 +</box>
 +<box 40% left #f0f0f0>
 +Equivalent circuit when the diode D1 is conducting and D2 blocking 
 +[[file/current_doubler_d1_magnetica.png|{{current_doubler_d1_magnetica.png}}]]
 +{{page>insert/by_SZ}}
 +</box>
 +<box 40% left #f0f0f0>
 +Equivalent circuit when the diode D2 is conducting and D1 blocking
 +[[file/current_doubler_d2_magnetica.png|{{current_doubler_d2_magnetica.png}}]]
 +{{page>insert/by_SZ}}
 +</box>
 +
 +===== See also =====
 +  *[[Push-pull]]
 +  *[[Centre tap]]
 +  *[[List of SMPS topologies]]
 +
 +===== References =====
 +~~REFNOTES~~
 +
 +{{tag>Inductors Switch-mode_power_supplies Current_doublers Counter}}
  

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